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Characteristics of electronic visual analogue and numerical scales for ratings of experimental pain in healthy subjects and fibromyalgia patients
Pain 140 (2008) 158–166 www.elsevier.com/locate/pain
Characteristics of electronic visual analogue and numerical scales for ratings of experimental pain in healthy subjects and fibromyalgia patients Donald D. Price a,b, Rahul Patel c, Michael E. Robinson d, Roland Staud c,* a
Department of Neuroscience, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA Department of Oral Surgery, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA c Department of Medicine, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA d Department of Clinical & Health Psychology, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA b
Received 31 January 2008; received in revised form 29 July 2008; accepted 30 July 2008
Abstract Comparisons of measurement characteristics were made for three types of electronic pain scales: (a) visual analogue scale (VAS), (b) VAS combined with an electronic number box (VAS-N; 0–100), and (c) electronic number box scale (NUM). The three scales were capable of discriminating pain sensations from very small (0.5 °C) temperature steps in 13 healthy males, 26 healthy females, and 16 female fibromyalgia (FM) patients. All scales provided monotonic functions when used by subjects to rate pain from 5 s nociceptive temperatures (45–49 °C), thereby demonstrating the generality of these results across different demographic groups. As expected, FM patients rated heat pain sensations higher on all scales in comparison to healthy females, demonstrating the capacity of these scales to detect well-established group differences in pain sensitivity that exist across these two groups. However, in comparison to male subjects, healthy females gave higher NUM but not VAS or VAS-N ratings to the range of nociceptive presented temperatures. We interpret this difference as a selective scaling bias of female subjects for NUM. Finally, all three groups (total of 55 subjects) found the scales easy to use after brief instructions, though subjects strongly preferred the use of VAS-N or VAS in comparison to NUM scale. Ó 2008 Published by Elsevier B.V. on behalf of International Association for the Study of Pain. Keywords: Pain scales; Scaling; Visual analogue scale; Ratio scales; Psychophysics; Fibromyalgia; Sex differences
1. Introduction Despite the fact that visual analogue (VAS) and numerical rating scales (NUM) have been used for decades to measure pain, there is no consensus as to which type of scale most closely fulfills criteria for ideal measurements. Whereas VAS have been shown to have ratio scale properties [13,14,17,18], demonstrate high repeatability [20,22] and test–retest reliability [11,15], studies *
Corresponding author. Tel.: +1 352 273 5346; fax: +1 352 392 8483. E-mail address: staudr@ufl.edu (R. Staud).
of their use in acute pain settings generally find that the standard pencil-and-paper VAS requires greater cognitive capacity as well as motor skills and is more prone to errors than NUM [7,8]. Scoring was improved with the introduction of the mechanical [17] and electronic VAS [5]. Despite its ease of use, NUM clearly does not have ratio scale characteristics [17] and its ability to discriminate small differences is more limited than VAS [8,9]. Although electronic and mechanical VAS are easy to score, the extent to which they are userfriendly and accepted by healthy volunteers and patients has not been formally documented. Some patients and healthcare providers may be reluctant to utilize pain
0304-3959/$34.00 Ó 2008 Published by Elsevier B.V. on behalf of International Association for the Study of Pain. doi:10.1016/j.pain.2008.07.028
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scales without numbers. If combining numerical and visual operations during the use of VAS do not change the measurement properties of the VAS, then adding numbers may facilitate the ease of the use of VAS and may improve their measurement characteristics. In light of these considerations, the purpose of this study was to compare measurement characteristics and subject preferences for three electronic pain scales (Fig. 1): (1) electronic VAS; (2) electronic VAS combined with a number box (VAS-N) (similar to the loudness scale on some televisions), and (3) a number box (NUM) without the VAS (i.e., a 0–100 numerical rating scale). The three scales were compared for psychometric characteristics, including the extent to which these three scales can discriminate small differences in experimental
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pain intensity and whether either 0–100 point NUM or VAS-N result in a systematic scaling bias, as was previously found for 0–10 NUM [17]. It has been previously shown that the mechanical VAS could reliably discriminate pain from contact thermode temperatures 1 °C apart [22]. Since human capacity for discriminating small differences in pain is about 0.2 °C [2], a more stringent test needs to be made for these three types of pain scales. Thus, these psychometric characteristics were compared across three types of electronic scales. Furthermore, these tests were made in healthy male and female subjects and in female fibromyalgia (FM) patients in order to determine whether psychometric properties of the scales were influenced by demographic characteristics and to extend the generality of our findings. 2. Materials and methods
Panel A Most Intense Pain Sensation Imaginable
No Pain at All
Panel B Most Intense Pain Sensation Imaginable
No Pain at All
45
Panel C 100 = Most Intense Pain Sensation Imaginable
0 = No Pain at All
45
Normal control (NC) subjects came from the University Health Science Center and the University Campus, Gainesville. Subjects who fulfilled the 1990 American College of Rheumatology (ACR) Criteria for FM were recruited from the Health Science Center Outpatient Clinics and from FM support groups. Informed consent was obtained from all subjects and the study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki. The University of Florida Institutional Review Board approved the procedures and protocol for this study. Prior to testing, all subjects underwent a clinical examination and were excluded from the study if they had abnormal findings. Abnormal findings which resulted in exclusion included but were not limited to painful osteoarthritis, peripheral neuropathies, and skin changes that interfered with heat sensory testing. By definition FM patients had to have mechanical hyperalgesia/allodynia. Use of analgesics, including nonsteroidal anti-inflammatory drugs (NSAID) and acetaminophen, was not allowed during the study. All subjects were asked to discontinue analgesics for the duration of five drug half-lives before testing, except narcotics which had to be stopped at least two weeks prior to study entry. Low dose muscle relaxants and/ or amitriptyline (610 mg/day) were permissible during the study for treatment of FM-related insomnia. 2.1. Experimental design
Fig. 1. Electronic pain scales used for the experiments. Participants rated experimental pain on three scales anchored on the left by ‘‘no pain at all” and on the right by ‘‘most intense pain sensation imaginable”. Individual scales were displayed on a 17 in. liquid crystal display (LCD) monitor. Participants rated painful sensations by rotating a computer track-ball which extended a red bar from left to right. When numbers were displayed in a box (B and C), rotation of the track-ball advanced the numbers from 0 to 100 in 1 step intervals. (A) Visual analogue scale (VAS); (B) VAS plus numbers (VAS-N); (C) numerical scale (NUM).
Experimental pain was elicited by phasic heat pulses (see below) to the volar surface of each forearm. Five seconds heat stimuli between 45 and 49 °C (see Section 2.3) were applied to three different areas of each forearm separated by 10 cm in counterbalanced order. A Peltier thermode (see Section 2.4) was used to apply the heat stimuli to the forearms. The inter-stimulus interval was at least 1 min or until all pain sensations had subsided.
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2.2. Ratings of pain
would be represented by ratings near the left, middle, and right portions of the scale, respectively”. After a brief training period all study subjects were comfortable with the three different scales alternatively using either their left or right hand to manipulate the VAS, VAS-N or NUM. All scaling experiments were applied in counterbalanced order to avoid order effects.
2.2.1. Ratings of experimental pain Three different horizontal electronic scales were used for pain ratings during the heat pain experiments: (1) visual analogue scale (VAS); (2) VAS combined with number box (VAS-N); (3) number box numerical scale (NUM). All scales ranged from 0 to 100 using intervals of 1. The scales were anchored on the left by: ‘‘no pain sensation” and on the right by: ‘‘the most intense pain imaginable”. All electronic scales were developed by one of the authors (R. Staud) using LabView 7.1 (National Instruments Corporation, Austin, TX). Each pain scale was displayed on a 17 in. liquid crystal display (LCD) computer screen (see Fig. 1). On a second LCD screen, not visible to the study subject, the experimenters could observe the numerical results of each pain rating. A computer track-ball was used by the study participants to move the slider of the VAS and VAS-N scales revealing a red horizontal bar. For numerical pain scaling the number box display changed incrementally from 0 to 100 by moving the track-ball. As described before [17] though slightly modified for the current protocol, all subjects were instructed that there are two aspects of pain: the intensity of painful sensation and how unpleasant or disturbing it is to have the pain. Each subject received the following verbal instructions: ‘‘The distinction between the intensity and unpleasantness of pain might be made clearer if you think of listening to a sound, such as that coming from a radio. As the volume of the sound increases, we can ask you how loud it sounds or how unpleasant it is to you. The intensity of painful sensations is like loudness; the unpleasantness of pain depends on its intensity and other things that may influence your estimation of unpleasantness. There are two scales for measuring each of these two aspects of pain. During this experiment, however, we would like you to only rate the intensity of your experimental heat pain. Please manipulate the track-ball to move the scale to the right, for pain intensity; the farther to the right, the greater the pain sensation intensity. The extreme left means no pain sensation at all and the extreme right indicates a pain sensation intensity that you imagine is the most intense that you could possibly experience. Thus mild, moderate, and intense pain sensations
10C 450C
10 C 460C
2.2.2. Ratings of somatic pain A mechanical visual analogue scale (0–10) was used for ratings of somatic pain before and after the experimental protocol [17]. Although the NC subjects were required to be pain free at enrollment somatic pain ratings were obtained before and after the testing session to capture new incidental pains such as back aches and headache. The scale was anchored on the left with ‘‘no pain at all” and on the right with ‘‘the most intense pain imaginable”. 2.3. Heat stimuli Five seconds ramp and hold heat pulses were used as experimental pain stimuli. Each stimulus train was comprised 5 heat stimuli ranging from 45 to 49 °C (in 1 °C steps). Additionally, 47.5 °C stimuli were used to test the sensitivity of the pain scales to small increments in stimulus intensity. This design provided two 0.5 °C steps (from 47 to 47.5 °C and from 47.5 to 48 °C) to the protocol (Fig. 2). At the end of each 5 s heat stimulus the participants were immediately asked to rate the intensity of their painful sensations using one of the pain scales (VAS, VAS-N, NUM). The heat stimuli were always applied in the following order: (a) ascending; (b) descending; and (c) random. Thus the participants underwent three different stimulus trains (ascending, descending, and random) during testing of each pain scale. This design resulted in 54 heat pulses for each participant. Due to the large number of stimuli and to prevent sensitization of either forearm each stimulus sequence was applied only once. 2.4. Thermal probe During the experiments a Peltier thermode with a contact surface of 3 3 cm (9 cm2) (TSA-2001, Medoc
0.50C 470C
0.50C
47.50C
480C
10C 490C
Fig. 2. Characteristics of heat stimuli applied to NC-F, NC-M, and FM subjects. Each participant underwent heat pain testing on both forearms. Five seconds ‘ramp and hold’ heat stimuli ranging from 45 to 49 °C were tested in 1 °C steps and in counterbalanced order at six different areas of the participants’ volar forearms (3 on each arm). To test the sensitivity of the scales a 0.5 °C step was included (47.5 °C).
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Advanced Medical Systems, Ramat Yishai, Israel) was used for the thermal stimuli. For heat pain testing the preheated probe was brought into firm contact with the skin of the volar forearm for 5 s.
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[26]. All participants were right handed and included 47 Caucasians, one African–American and seven Asian subjects. 3.1. Clinical pain ratings
2.5. Scale preferences All participants were asked at the end of the psychophysical experiments to rank order the three scales according to the following questions: Which of the three scales did you prefer most for rating of your pain sensations? Please rank order your choices from most to least preferred. If you cannot make up your mind, use ‘‘can’t decide”. 2.6. Tender point testing All subjects underwent tender point testing as defined by the American College of Rheumatology (ACR) Criteria [26]. For this purpose nine paired tender points and two control points (at the center of the right forearm and the right thumbnail) were assessed by a trained investigator using a Fischer Dolorimeter (Pain Diagnostics, Great Neck, NY). The rubber tip of the Dolorimeter was 1 cm in diameter. The Dolorimeter was placed on the examination site, and pressure was gradually increased by 1 kg/s. The subjects were instructed to report when the sensation at the examination site changed from pressure to pain. Pressure testing was stopped at that moment and the result recorded as positive (1) if maximal pressure was 64 kg. If no pain was elicited at P4 kg the test result was recorded as negative (0). 2.7. Statistical analysis Statistical comparisons utilized SPSS 15.0 software (SPSS, Inc., Chicago, IL). Because of our interest in the discriminability of the scales for 0.5 and 1 °C steps two separate sets of mixed model ANOVAs were performed. The significance level was set at .05. Scales (3) and temperature (5) were the within subjects’ factors, and diagnosis (2) or sex (2) were the between subjects’ factor. Simple contrasts were used to decompose significant interactions effects. To test differences in scale preferences reported by the study subjects, z-tests for proportions were conducted comparing all scales. 3. Results We recruited 39 healthy subjects (females: 26; males 13) [mean age (SD): 35.3 (13.3) years] using advertisements posted throughout the University of Florida, Gainesville and 16 female FM patients [52.0 (12.0) years] from the local community and FM support groups. The chronic pain patients fulfilled the 1990 American College of Rheumatology Criteria for FM
Mean (SD) clinical pain rating [mean (SD); mechanical VAS scale: 0–10] of NC and FM subjects was 0.6 (0.6) VAS units and 3.6 (3.2) VAS units, respectively. An independent t-test indicated significantly higher pain ratings of FM subjects compared to NC (t(57) = 6.4; p < .001). There was no significant difference in clinical pain ratings between NC-F and NC-M (p > .05). 3.2. Comparisons of scaling properties of VAS, VAS-N, and NUM Statistical comparisons between ratings of ascending, descending, and random heat stimuli showed a significant difference between random and both other series (F(2, 74) = 5.2; p = .008). Study subjects rated random stimuli lower than either ascending or descending heat pulses. Ratings of ascending and descending series were not statistically different (p > .05). Because the statistical analysis of individual pain ratings during ascending, descending, and random series showed systematic differences (see Section 3.2), average ratings for each stimulus across the three scales were computed to achieve a reliable estimate. 3.2.1. Time spent to familiarize subjects with scale usage The total time spent to instruct each participant on the pain scaling methods used for the experiments was 80 s. Time spent to familiarize the subjects with (a) rating of pain intensity according to standardized instructions [17] was 30 s; (b) how to use of the electronic VAS: 15 s; (c) how to use of the electronic VAS-N: 20 s; (d) how to use of the electronic NUM scale: 15 s. All subjects reported to be comfortable with the three pain scales and were able to complete all pain scaling procedures. 3.2.2. Experimental pain ratings of heat stimuli using 1 °C increments 3.2.2.1. Pain ratings of 45–49 °C stimuli by NC-F and NC-M subjects using VAS, VAS-N or NUM scale. For all three scales, experimental pain ratings of NC-F and NC-M subjects increased monotonically for stimuli between 45 and 49 °C (see Fig. 3). A mixed model ANOVA with scales (3) and stimulus temperature (5) as within and sex (2) as between subjects’ factors showed a significant effect for temperature (F(4, 148) = 91.4; p < .001) and scales (F(2, 74) = 3.5; p = .034), without significant interaction effects of scale temperature (p > .05), indicating that all scales registered similar rate of increases in pain associated with increasing stimulus
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45 C
46 C
p < .001
47 C
48 C
49 C
p < .02
70
p < .04
p < .02
p < .01
60 ns
ns
Pain Ratings (0-100)
50 40 30 20 10 0 F
M
FM
F
VAS
M VAS -N
FM
F
M
FM
NUM
Fig. 3. Average (SE) experimental pain ratings (45–49 °C heat pulse trains; 1 °C steps) by NC-F, NC-M, and FM subjects using VAS, VAS-N, and NUM scales. All scales were able to discriminate pain sensations related to 1 °C increases in temperature regardless of diagnostic group (p < .001). For all scales except NUM, pain ratings did not statistically differ between NC-F and NC-M (p > .05). Using NUM, NC-F provided higher experimental pain ratings than NC-M (p < .01). FM subjects rated heat pulses significantly more painful than NC, regardless of scale usage (p < .04– p < .001).
intensity. There was also no sex difference noted for these monotonic increases of pain ratings (F(1, 37) = .8; p > .05). Use of simple contrasts indicated that pain ratings of subjects were higher using the NUM scale than the other two scales (F(1, 37) = 5.2; p = .03). In addition, the analysis showed a significant scale sex interaction effect (F(2, 74) = 5.6; p = .005). Further analysis revealed that NC-F rated experimental pain higher than NC-M using the NUM scale (F(1, 37) = 9.0; p = .005) but not while using the VAS or VAS-N (all p > .05). Thus, all three pain scales showed similar increases in pain ratings related to 1 °C steps in stimulus temperatures. The NUM scale, however, provided higher ratings compared to VAS and VAS-N, in particular for female subjects. The mean differences in NC subjects’ experimental pain ratings across the three scales are shown in Table 1. Most of the rating differences across scales were small, except for VAS vs Table 1 Differences scores of experimental pain ratings across three scales using the grand average of all stimuli DVAS NC-F (SD) FM-F (SD) All females (SD) NC-M (SD)
VAS-N
DVAS
NUM
DVAS-N
NUM
1.31 (5.1) 1.10 (7.4) 0.45 (6.0)
7.75 (12.1) 4.39 (6.3) 6.41 (10.3)
9.06 (14.0) 3.28 (7.2) 6.86 (12.1)
4.49 (10.8)
3.45 (10.3)
1.04 (7.8)
VAS, visual analogue scale (0–100); VAS-N, VAS with number box (0–100); NUM, numerical scale (number box) (0–100).
NUM and VAS-N vs NUM. Here the mean (SD) difference of NC-F pain ratings was 7.5 (12.1) and 9.06 (14.0) exceeding NC-M by a factor of more than two. 3.2.2.2. Pain ratings of 45–49 °C stimuli by NC-F and female FM subjects using VAS, VAS-N or NUM scale. Similar to experimental pain ratings of NC-M and NC-F subjects the ratings of FM subjects increased monotonically with increasing stimulus temperatures (see Fig. 3). Data from NC-F and female FM subjects were compared using a mixed model ANOVA with scales (3) and stimulus temperature (5) as within and diagnosis (2) as between subjects’ factors. This analysis showed main effects for scale (F(2, 80) = 10.2; p < .001), temperature (F(4, 180) = 126.3; p < .001), and diagnosis (F(1, 40) = 8.9; p = .005). There were significant interaction effects for temperature diagnosis (F(4, 160) = 3.0; p = .02) and scale temperature (F(8, 320) = 3.5; p = .001) noted. The interaction between scale and diagnosis was not significant (F(2, 80) = 1.7; p > .05). To decompose the significant interaction effects, simple contrasts were used. The results showed that the NUM scale provided higher pain ratings than the VAS (F(1, 40) = 13.1; p = .001) or the VAS-N (F(1, 40) = 10.7; p = .002). In addition, FM subjects provided higher pain rating for each temperature step compared to NC (p = .02–04). These results indicate that similar to healthy controls, FM subjects’ experimental pain ratings monotonically increased with increasing stimulus temperatures, albeit at different
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rates. Additionally, the FM subjects rated experimental pain higher using the NUM scale than the VAS or VASN. The absolute difference in female subjects’ experimental pain ratings across the three scales is shown in Table 1. Thus, independent of diagnosis, all female subjects could separately distinguish the pain associated with step-wise increases in temperature and they seemed to similarly use the three pain scales.
pared to NC-M, NC-F provided higher pain ratings using the NUM scale than using the VAS (F(1, 37) = 7.7; p = .009) or VAS-N (F(1, 37) = 4.7; p = .04). Using only NUM ratings, a mixed model ANOVA of temperatures (3) and sex (2) showed a main effect for sex (F(1, 37) = 80.0; p = .04), indicating that NC-F rated experimental pain higher than NC-M using this scale.
3.2.3. Experimental pain ratings of heat stimuli using 0.5 °C increments To evaluate the sensitivity of the three scales to change, pain rating associated with small increments in temperature (0.5 °C steps) were obtained from NC-F and NC-M as well as female FM subjects.
3.2.3.2. Pain ratings of 47–47.5–48 °C stimuli by NC-F and female FM subjects using VAS, VAS-N or NUM scale. Similar to NC participants, female FM subjects reported increasing pain intensity during 0.5 °C temperature increments between 47 and 48 °C (see Fig. 4). A mixed model ANOVA was used to analyze the subjects’ pain ratings with scale (3) and temperature (3) as within and group (2) as between subjects’ factors. There were significant main effects for scale (F(2, 80) = 6.3; p = .003), temperature (F(2, 80) = 43.4; p < .001), and group (F(1, 40) = 4.5; p = .04) noted. However, there were no significant interaction effects present for scale group, temperature group, or scale temperature (all p > .05). This lack of interactions indicates that small temperature increments resulted in similar rating increases in all groups and for all scales. The main effect for temperature indicates that the slopes for both stimulus increments were non-zero, an indication that participants could discriminate between 0.5° and 1° increments. Use of simple contrasts showed that (a) the NUM scale provided higher pain ratings than the VAS (F(1, 40) = 6.3; p = .02) or the VAS-N (F(1, 40) = 8.3; p = .006); (b) VAS or VAS-N ratings did not statistically differ within either group of subjects (p > .05); (c) all scales could distinguish pain ratings
3.2.3.1. Pain ratings of 47–47.5–48 °C stimuli by NC-F and NC-M subjects using VAS, VAS-N or NUM scale. The results indicate that for both groups all three scales provided monotonic increases of pain ratings with increasing stimulus temperature using 0.5 °C steps (see Fig. 4). A mixed model ANOVA was used to analyze the data with scale (3) and temperature (3) as within and sex (2) as between subjects’ factors. This analysis showed main effects for temperature (F(2, 74) = 36.4; p < .001) but not for scale (p > .05) or sex (p > .05). There was, however, a significant scale sex interaction effect (F(2, 74) = 5.1; p = .008) noted. Interactions of temperature sex (p > .05) or scale temperature (p > .05) were not significant. The lack of significant temperature scale interaction effect indicates that every scale could distinguish pain ratings related to 0.5 °C increments. Simple contrasts were used to decompose the scale sex interaction effect showing that com-
47 C
70
47.5 C
p < .001
p < .04
p < .02
60 Pain Ratings (0-100)
48 C
50
p < .02
p = .04 ns
ns
40 30 20 10 0 F
M VAS
FM
F
M V A S -N
FM
F
M
FM
NUM
Fig. 4. Average (SE) experimental pain ratings of NC-F, NC-M, and FM subjects during 47–48 °C heat pulse trains (0.5 °C steps) using VAS, VASN, and NUM scales. All scales were able to discriminate pain sensations related to 0.5 °C increases in temperature regardless of diagnostic group (p < .001). Except NUM (p = .42), pain ratings did not statistically differ between NC-F and NC-M (p > .05). Using NUM, NC-F provided higher experimental pain ratings than NC-M using either the VAS or VAS-N (p < .01). FM subjects rated heat pulses significantly more painful than NC, regardless of scale usage (p < .04–p < .001).
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related to small increments in heat stimuli (F(1, 40) = 17.9; p < .001). A follow-up analysis using t-tests to compare adjacent stimulus pairs showed that 55 of 59 adjacent pairs were highly statistically different, corroborating the ANOVA results. The vast majority (at both 0.5° and 1°) were highly significant (p < .001). 3.3. Scale preferences Most study subjects were able to rank order the three pain scales according to their personal preferences. Only 4 NC (10.3%) and 1 FM (6.3%) subject could not provide a scale preference. The VAS-N was the most preferred pain scale by 43.6% of NC and 50% of FM subjects followed by the VAS over all other scales (see Table 2). The electronic NUM was by far the least preferred scale. To test the significance of these differences we conducted z-tests for proportions comparing each VAS scale (VAS, VAS-N) to NUM and combined VAS (VAS = VAS-N) to NUM. The proportion of subjects preferring the VAS to NUM was significant at the 90% confidence interval. The proportion preferring VAS-N to NUM was significant at the 95% confidence interval and the combined VAS vs. NUM was significant at the 99% confidence interval. These analyses seem to clearly indicate a preference for the VAS scales over NUM scales. The rank order of the three pain scales was the same for NC and FM subjects (1 = VAS-N; 2 = VAS; 3 = NUM). 4. Discussion The characteristics of three types of electronic pain scales (VAS, VAS-N, and NUM) were tested in NC as well as in FM patients. Comparisons of measurement show that all scales were capable of discriminating experimental pain ratings related to 0.5 and 1 °C temperature steps within the nociceptive range. In addition, the pain ratings of NC-M, NC-F, and female FM patients followed very similar monotonic functions, thereby demonstrating the generality of results across different demographic groups. As expected, FM patients rated identical heat stimuli higher on all scales in comparison to NC subjects, demonstrating the capacity of these scales to detect group differences in pain sensitivity known to exist across FM and NC groups [19,23]. However, NC-F subjects gave higher NUM ratings than NCM subjects, but not higher VAS or VAS-N ratings to the Table 2 Scale preferences of study participants Group (n)
VAS (%)
VAS-N (%)
NUM (%)
Cannot decide (%)
NC (39) FM (16)
14 (36.0) 6 (37.4)
17 (43.6) 8 (50.0)
4 (10.2) 1 (6.3)
4 (10.2) 1 (6.3)
n, number of subjects; VAS, visual analogue scale; VAS-N, VAS with number box; NUM, numerical scale (number box).
range of noxious temperatures presented. Finally, after brief instructions, all three groups (total of 55 study subjects) found the scales easy to use, though most participants preferred the VAS-N or VAS over the NUM scale. In an exploratory analysis random heat stimuli were found to result in lower pain ratings than ascending or descending heat pulses. Because our study was not designed to explore this very interesting finding in more detail, future investigations will be necessary. To obtain a representative sample for our analysis, we combined the ratings of the three different heat pulse series. 4.1. Scale similarities The capacity of all three scales to discriminate ratings related to small (0.5 °C) stimulus differences is surprising, especially since the NUM has been claimed to provide only 10 pain levels [9,10]. This sensitivity to detect change is probably even greater than reported here, as suggested by the highly significant differences (p < .001) and robust effects sizes (partial g2 = .3 to .6) between adjacent 0.5 °C steps. The limit of discrimination for heat induced pain is 0.2–0.3 °C [2]. Rosier et al. [22] showed that mechanical VAS (M-VAS) could easily discriminate 1 °C steps over a range of 43–49 °C. If the capacity to discriminate small differences is equal to 0.5 °C and is similar over most or all of the 0–100 U range, then these scales clearly can discriminate greater than 10 levels of pain and have greater discriminatory power than previously believed. All three scales, including the 0–100 NUM, are likely to provide better discrimination than that described for the 0–10 NUM [9]. However, the specific use of the NUM in our study, including the track-ball manipulations that subjects were required to use, may make this scale different than a typical 0–10 NUM. Regardless of the reasons for the differences, this study is the first to demonstrate the capacity of three different types of electronic pain scales to detect such small differences in pain intensity. This capacity is highly relevant for pain measurement in both experimental and clinical settings. The capacity to detect pain changes initiated by pharmaceutical or other types of therapeutic/experimental interventions is critical for a study’s assay sensitivity, particularly the detection of sometimes small but systematic effects. The electronic VAS has been previously shown to provide positively accelerating power functions for groups as well as for individual subjects [15] and these functions strongly resemble those obtained in previous studies that used either standard or mechanical VAS [17,18]. The electronic VAS has demonstrated very high test–retest reliability (r = .88 to .99) and to accurately measure heritability of cold pressor (=60%) and contact heat-evoked pain (=26%) [16]. Importantly, measurement error accounted for only 5–8% of the variance in pain ratings [15,16]. Provided that adequate instructions
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were given, individual differences in electronic VAS ratings of pain mainly reflect actual differences in experienced pain intensity, not differences in scale use. Thus, these results add to the validation of the scales used in this study. 4.2. Does adding a number box to the electronic VAS change its measurement properties? The stimulus–response functions of VAS and VAS-N were very similar in our study. Both scales were able to discriminate small (0.5 °C) temperature steps and could detect large differences in pain sensitivity between FM and NC groups, as previously shown [19]. Although we are not recommending the VAS-N over VAS our results show that adding a number box to an electronic VAS does not seem to significantly change its measurement properties. However, adding a number box may not be comparable to adding numbers to a standard VAS, because a subject can simultaneously view all the numbers on the latter type scale. The electronic VASN provides a box, wherein the numbers increase monotonically as the red bar moves from left to right. Thus, a practical implication of this study is to use a VAS-N for those patients and healthcare practitioners who are uncomfortable with pain scales devoid of numbers, since adding a number box made no significant change in their psychometric properties. Furthermore, according to the study subjects, the VAS-N was the most preferred scale. 4.3. Does use of the 0–100 NUM scale produce a scaling bias? A previous study of orofacial pain patients (21$; 2#) showed that the mechanical VAS but not 0–10 NUM had ratio scale properties [17]. Unlike VAS, the 0–10 NUM did not display a distinct zero point (see Fig. 4 and associated data in Price et al. [17]) and 0–10 NUM ratings were found to be artificially higher for both clinical and experimental pain than VAS ratings [6,17,24]. Although both scales generate monotonic stimulus response functions, the 0–10 NUM function is displaced to the left of the VAS stimulus–response curve [17]. A consequence of this displacement of the 0–10 NUM stimulus–response function is the loss of ratio scale properties. This shift also produces artificially high ratings near pain threshold, making it less likely for NUM to have a true zero point [17]. In contrast, VAS ratings for 45 °C stimuli are close to zero. Given these differences, the 0–10 NUM almost certainly contains a scaling bias and cannot provide the same information as the VAS, though their ratings may be strongly correlated. This study shows that this scaling bias occurred only for women. This may be clinically relevant for healthcare providers if they want to assess percentage
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change in pain. The lack of NUM’s ratio scale properties would certainly introduce a systematic error in such pain measurements. The results of this study further confirm and extend our previous comparison of VAS and 0–10 NUM [17]. Similar to the previous study, female subjects gave higher NUM ratings than VAS ratings (Fig. 3 and Table 1). Although these differences were small [up to 9.02 units (scales: 0–100)] they were not negligible. Importantly, almost all of these increased pain ratings were provided by female but not by male subjects (Fig. 3 and Table 1). These differences were not found between VAS and VAS-N scales. There are two possible interpretations of these findings. One is that the 0–100 NUM is the only one capable of detecting sex differences in pain sensitivity and the other is that this scale fosters a scaling bias predominantly in female subjects. We think the latter interpretation is correct for two reasons. First, VAS has been able to detect robust sex differences in pain sensitivity in human studies [21,25]. For example, Robinson et al. [21] found reliable sex effects for cold pressor pain (Cohen’s d = .4 to .7) using the electronic VAS. Sex differences vary according to experimental pain models, ranging from large effects for cold pressor tests to very small or negligible effects for heat pulses like those used in this study [4,16,25]. Second, recent studies have shown the electronic VAS to be sensitive to individual and group differences in general [15,16]. Finally, contrary to several studies that compare VAS and NUM in clinical settings wherein acute pain is measured [1,3,6,12], our study subjects strongly preferred VAS or VAS-N to NUM (Table 2) and all the 55 subjects were able to use these scales after brief standardized instructions. Because the ratings were registered automatically, these results were free of scoring errors. Until now the user-friendliness of VAS was controversial among researchers and clinicians. The reluctance of some healthcare providers and patients to use the VAS in clinical practice has resulted in the widespread use of pain scales with inferior scaling properties (e.g. 0–10 NUM and Faces Scales with only two sad faces). In contrast, mechanical and electronic VAS have excellent psychometric properties and greatly reduce difficulties claimed to exist for standard VAS (higher cognitive demand and recording errors). 5. Conclusions The combination of results from this investigation and previous studies show that the electronic VAS has excellent pain measurement properties, including the capacity to measure very small differences. Adding a number box does not seem to change these properties significantly. However, a number box alone (NUM) appears to introduce a scaling bias, predominantly in female subjects (Table 1). Unlike many previous studies,
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the electronic VAS and VAS-N were easy to administer and score and most strongly preferred by subjects. Overall, the electronic VAS has psychometric properties that are strongly superior to 0–10 NUM and are likely to be better than the 0–100 NUM. Given the superior psychometric characteristics of VAS (mechanical and electronic) over NUM, these scales can be highly recommended for clinical research and practice.
Acknowledgments Supported by NIH Grants NS-38767 and AR053541. The expert technical assistance of Susann Nagel is gratefully acknowledged. None of the authors had any financial interests in the scales that were developed for this study.
References [1] Breivik EK, Bjornsson GA, Skovlund E. A comparison of pain rating scales by sampling from clinical trial data. Clin J Pain 2000;16:22–8. [2] Bushnell MC, Taylor MB, Duncan GH, Dubner R. Discrimination of innocuous and noxious thermal stimuli applied to the face in human and monkey. Somatosens Res 1983;1:119–29. [3] Ekblom A, Hansson P. Pain intensity measurements in patients with acute pain receiving afferent stimulation. J Neurol Neurosurg Psychiatry 1988;51:481–6. [4] Fillingim RB, Edwards RR, Powell T. The relationship of sex and clinical pain to experimental pain responses. Pain 1999;83:419–25. [5] Gerson GR, Gerson JN. Assessment of pain using a computer. Anaesthesia 1980;35:815–7. [6] Jensen MP. The validity and reliability of pain measures for use in clinical trials in adults: review paper written for the Intiative on Methods, Measurements, and Pain Assessment in Clinical Trials (IMMPACT) meeting, April 12–13 (2003). IMMPACT-II; 2003. [7] Jensen MP. The validity and reliability of pain measures in adults with cancer. J Pain 2003;4:2–21. [8] Jensen MP, Karoly P. Self-report scales and procedures for measuring pain in adults. In: Turk D, Melzack R, editors. Handbook of pain assessment. New York: Guilford Press; 2001. p. 15–34. [9] Jensen MP, Turner JA, Romano JM. What is the maximum number of levels needed in pain intensity measurement? Pain 1994;58:387–92.
[10] Jensen MP, Turner JA, Romano JM, Fisher LD. Comparative reliability and validity of chronic pain intensity measures. Pain 1999;83:157–62. [11] Kiernan BD, Dane JR, Phillips LH, Price DD. Hypnotic analgesia reduces R-III nociceptive reflex: further evidence concerning the multifactorial nature of hypnotic analgesia. Pain 1995;60:39–47. [12] Kremer E, Atkinson JH, Ignelzi RJ. Measurement of pain: patient preference does not confound pain measurement. Pain 1981;10:241–8. [13] Myles PS, Troedel S, Boquest M, Reeves M. The pain visual analog scale: is it linear or nonlinear? Anesth Analg 1999;89:1517–20. [14] Myles PS, Urquhart N. The linearity of the visual analogue scale in patients with severe acute pain. Anaesth Intensive Care 2005;33:54–8. [15] Nielsen CS, Price DD, Vassend O, Stubhaug A, Harris JR. Characterizing individual differences in heat-pain sensitivity. Pain 2005;119:65–74. [16] Nielsen CS, Stubhaug A, Price DD, Vassend O, Czajkowski N, Harris JR. Individual differences in pain sensitivity: genetic and environmental contributions. Pain 2008;136:21–9. [17] Price DD, Bush FM, Long S, Harkins SW. A comparison of pain measurement characteristics of mechanical visual analogue and simple numerical rating scales. Pain 1994;56:217–26. [18] Price DD, McGrath PA, Rafii A, Buckingham B. The validation of visual analogue scales as ratio scale measures for chronic and experimental pain. Pain 1983;17:45–56. [19] Price DD, Staud R, Robinson ME, Mauderli AP, Cannon RL, Vierck CJ. Enhanced temporal summation of second pain and its central modulation in fibromyalgia patients. Pain 2002;99: 49–59. [20] Quiton RL, Greenspan JD. Across- and within-session variability of ratings of painful contact heat stimuli. Pain 2008;137:245–56. [21] Robinson ME, Gagnon CM, Riley JL, Price DD. Altering gender role expectations: effects on pain tolerance, pain threshold, and pain ratings. J Pain 2003;4:284–8. [22] Rosier EM, Iadarola MJ, Coghill RC. Reproducibility of pain measurement and pain perception. Pain 2002;98:205–16. [23] Staud R, Koo E, Robinson ME, Price DD. Spatial summation of mechanically evoked muscle pain and painful after sensations in normal subject and fibromyalgia patients. Pain 2007;130:177–87. [24] Wallenstein SL, Heidrich III G, Kaiko R, Houde RW. Clinical evaluation of mild analgesics: the measurement of clinical pain. Br J Clin Pharmacol 1980;10:319S–27S. [25] Wise EA, Price DD, Myers CD, Heft MW, Robinson ME. Gender role expectations of pain: relationship to experimental pain perception. Pain 2002;96:335–42. [26] Wolfe F, Smythe HA, Yunus MB, Bennett RM, Bombardier C, Goldenberg DL. The American College of Rheumatology 1990 Criteria for the classification of fibromyalgia. Report of the multicenter criteria committee. Arthritis Rheum 1990;33:160–72.
Characteristics of electronic visual analogue and numerical scales for ratings of experimental pain in healthy subjects and fibromyalgia patients Donald D. Price a,b, Rahul Patel c, Michael E. Robinson d, Roland Staud c,* a
Department of Neuroscience, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA Department of Oral Surgery, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA c Department of Medicine, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA d Department of Clinical & Health Psychology, University of Florida, College of Medicine, Gainesville, FL 32610-0221, USA b
Received 31 January 2008; received in revised form 29 July 2008; accepted 30 July 2008
Abstract Comparisons of measurement characteristics were made for three types of electronic pain scales: (a) visual analogue scale (VAS), (b) VAS combined with an electronic number box (VAS-N; 0–100), and (c) electronic number box scale (NUM). The three scales were capable of discriminating pain sensations from very small (0.5 °C) temperature steps in 13 healthy males, 26 healthy females, and 16 female fibromyalgia (FM) patients. All scales provided monotonic functions when used by subjects to rate pain from 5 s nociceptive temperatures (45–49 °C), thereby demonstrating the generality of these results across different demographic groups. As expected, FM patients rated heat pain sensations higher on all scales in comparison to healthy females, demonstrating the capacity of these scales to detect well-established group differences in pain sensitivity that exist across these two groups. However, in comparison to male subjects, healthy females gave higher NUM but not VAS or VAS-N ratings to the range of nociceptive presented temperatures. We interpret this difference as a selective scaling bias of female subjects for NUM. Finally, all three groups (total of 55 subjects) found the scales easy to use after brief instructions, though subjects strongly preferred the use of VAS-N or VAS in comparison to NUM scale. Ó 2008 Published by Elsevier B.V. on behalf of International Association for the Study of Pain. Keywords: Pain scales; Scaling; Visual analogue scale; Ratio scales; Psychophysics; Fibromyalgia; Sex differences
1. Introduction Despite the fact that visual analogue (VAS) and numerical rating scales (NUM) have been used for decades to measure pain, there is no consensus as to which type of scale most closely fulfills criteria for ideal measurements. Whereas VAS have been shown to have ratio scale properties [13,14,17,18], demonstrate high repeatability [20,22] and test–retest reliability [11,15], studies *
Corresponding author. Tel.: +1 352 273 5346; fax: +1 352 392 8483. E-mail address: staudr@ufl.edu (R. Staud).
of their use in acute pain settings generally find that the standard pencil-and-paper VAS requires greater cognitive capacity as well as motor skills and is more prone to errors than NUM [7,8]. Scoring was improved with the introduction of the mechanical [17] and electronic VAS [5]. Despite its ease of use, NUM clearly does not have ratio scale characteristics [17] and its ability to discriminate small differences is more limited than VAS [8,9]. Although electronic and mechanical VAS are easy to score, the extent to which they are userfriendly and accepted by healthy volunteers and patients has not been formally documented. Some patients and healthcare providers may be reluctant to utilize pain
0304-3959/$34.00 Ó 2008 Published by Elsevier B.V. on behalf of International Association for the Study of Pain. doi:10.1016/j.pain.2008.07.028
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scales without numbers. If combining numerical and visual operations during the use of VAS do not change the measurement properties of the VAS, then adding numbers may facilitate the ease of the use of VAS and may improve their measurement characteristics. In light of these considerations, the purpose of this study was to compare measurement characteristics and subject preferences for three electronic pain scales (Fig. 1): (1) electronic VAS; (2) electronic VAS combined with a number box (VAS-N) (similar to the loudness scale on some televisions), and (3) a number box (NUM) without the VAS (i.e., a 0–100 numerical rating scale). The three scales were compared for psychometric characteristics, including the extent to which these three scales can discriminate small differences in experimental
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pain intensity and whether either 0–100 point NUM or VAS-N result in a systematic scaling bias, as was previously found for 0–10 NUM [17]. It has been previously shown that the mechanical VAS could reliably discriminate pain from contact thermode temperatures 1 °C apart [22]. Since human capacity for discriminating small differences in pain is about 0.2 °C [2], a more stringent test needs to be made for these three types of pain scales. Thus, these psychometric characteristics were compared across three types of electronic scales. Furthermore, these tests were made in healthy male and female subjects and in female fibromyalgia (FM) patients in order to determine whether psychometric properties of the scales were influenced by demographic characteristics and to extend the generality of our findings. 2. Materials and methods
Panel A Most Intense Pain Sensation Imaginable
No Pain at All
Panel B Most Intense Pain Sensation Imaginable
No Pain at All
45
Panel C 100 = Most Intense Pain Sensation Imaginable
0 = No Pain at All
45
Normal control (NC) subjects came from the University Health Science Center and the University Campus, Gainesville. Subjects who fulfilled the 1990 American College of Rheumatology (ACR) Criteria for FM were recruited from the Health Science Center Outpatient Clinics and from FM support groups. Informed consent was obtained from all subjects and the study protocol conformed to the ethical guidelines of the 1975 Declaration of Helsinki. The University of Florida Institutional Review Board approved the procedures and protocol for this study. Prior to testing, all subjects underwent a clinical examination and were excluded from the study if they had abnormal findings. Abnormal findings which resulted in exclusion included but were not limited to painful osteoarthritis, peripheral neuropathies, and skin changes that interfered with heat sensory testing. By definition FM patients had to have mechanical hyperalgesia/allodynia. Use of analgesics, including nonsteroidal anti-inflammatory drugs (NSAID) and acetaminophen, was not allowed during the study. All subjects were asked to discontinue analgesics for the duration of five drug half-lives before testing, except narcotics which had to be stopped at least two weeks prior to study entry. Low dose muscle relaxants and/ or amitriptyline (610 mg/day) were permissible during the study for treatment of FM-related insomnia. 2.1. Experimental design
Fig. 1. Electronic pain scales used for the experiments. Participants rated experimental pain on three scales anchored on the left by ‘‘no pain at all” and on the right by ‘‘most intense pain sensation imaginable”. Individual scales were displayed on a 17 in. liquid crystal display (LCD) monitor. Participants rated painful sensations by rotating a computer track-ball which extended a red bar from left to right. When numbers were displayed in a box (B and C), rotation of the track-ball advanced the numbers from 0 to 100 in 1 step intervals. (A) Visual analogue scale (VAS); (B) VAS plus numbers (VAS-N); (C) numerical scale (NUM).
Experimental pain was elicited by phasic heat pulses (see below) to the volar surface of each forearm. Five seconds heat stimuli between 45 and 49 °C (see Section 2.3) were applied to three different areas of each forearm separated by 10 cm in counterbalanced order. A Peltier thermode (see Section 2.4) was used to apply the heat stimuli to the forearms. The inter-stimulus interval was at least 1 min or until all pain sensations had subsided.
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2.2. Ratings of pain
would be represented by ratings near the left, middle, and right portions of the scale, respectively”. After a brief training period all study subjects were comfortable with the three different scales alternatively using either their left or right hand to manipulate the VAS, VAS-N or NUM. All scaling experiments were applied in counterbalanced order to avoid order effects.
2.2.1. Ratings of experimental pain Three different horizontal electronic scales were used for pain ratings during the heat pain experiments: (1) visual analogue scale (VAS); (2) VAS combined with number box (VAS-N); (3) number box numerical scale (NUM). All scales ranged from 0 to 100 using intervals of 1. The scales were anchored on the left by: ‘‘no pain sensation” and on the right by: ‘‘the most intense pain imaginable”. All electronic scales were developed by one of the authors (R. Staud) using LabView 7.1 (National Instruments Corporation, Austin, TX). Each pain scale was displayed on a 17 in. liquid crystal display (LCD) computer screen (see Fig. 1). On a second LCD screen, not visible to the study subject, the experimenters could observe the numerical results of each pain rating. A computer track-ball was used by the study participants to move the slider of the VAS and VAS-N scales revealing a red horizontal bar. For numerical pain scaling the number box display changed incrementally from 0 to 100 by moving the track-ball. As described before [17] though slightly modified for the current protocol, all subjects were instructed that there are two aspects of pain: the intensity of painful sensation and how unpleasant or disturbing it is to have the pain. Each subject received the following verbal instructions: ‘‘The distinction between the intensity and unpleasantness of pain might be made clearer if you think of listening to a sound, such as that coming from a radio. As the volume of the sound increases, we can ask you how loud it sounds or how unpleasant it is to you. The intensity of painful sensations is like loudness; the unpleasantness of pain depends on its intensity and other things that may influence your estimation of unpleasantness. There are two scales for measuring each of these two aspects of pain. During this experiment, however, we would like you to only rate the intensity of your experimental heat pain. Please manipulate the track-ball to move the scale to the right, for pain intensity; the farther to the right, the greater the pain sensation intensity. The extreme left means no pain sensation at all and the extreme right indicates a pain sensation intensity that you imagine is the most intense that you could possibly experience. Thus mild, moderate, and intense pain sensations
10C 450C
10 C 460C
2.2.2. Ratings of somatic pain A mechanical visual analogue scale (0–10) was used for ratings of somatic pain before and after the experimental protocol [17]. Although the NC subjects were required to be pain free at enrollment somatic pain ratings were obtained before and after the testing session to capture new incidental pains such as back aches and headache. The scale was anchored on the left with ‘‘no pain at all” and on the right with ‘‘the most intense pain imaginable”. 2.3. Heat stimuli Five seconds ramp and hold heat pulses were used as experimental pain stimuli. Each stimulus train was comprised 5 heat stimuli ranging from 45 to 49 °C (in 1 °C steps). Additionally, 47.5 °C stimuli were used to test the sensitivity of the pain scales to small increments in stimulus intensity. This design provided two 0.5 °C steps (from 47 to 47.5 °C and from 47.5 to 48 °C) to the protocol (Fig. 2). At the end of each 5 s heat stimulus the participants were immediately asked to rate the intensity of their painful sensations using one of the pain scales (VAS, VAS-N, NUM). The heat stimuli were always applied in the following order: (a) ascending; (b) descending; and (c) random. Thus the participants underwent three different stimulus trains (ascending, descending, and random) during testing of each pain scale. This design resulted in 54 heat pulses for each participant. Due to the large number of stimuli and to prevent sensitization of either forearm each stimulus sequence was applied only once. 2.4. Thermal probe During the experiments a Peltier thermode with a contact surface of 3 3 cm (9 cm2) (TSA-2001, Medoc
0.50C 470C
0.50C
47.50C
480C
10C 490C
Fig. 2. Characteristics of heat stimuli applied to NC-F, NC-M, and FM subjects. Each participant underwent heat pain testing on both forearms. Five seconds ‘ramp and hold’ heat stimuli ranging from 45 to 49 °C were tested in 1 °C steps and in counterbalanced order at six different areas of the participants’ volar forearms (3 on each arm). To test the sensitivity of the scales a 0.5 °C step was included (47.5 °C).
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Advanced Medical Systems, Ramat Yishai, Israel) was used for the thermal stimuli. For heat pain testing the preheated probe was brought into firm contact with the skin of the volar forearm for 5 s.
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[26]. All participants were right handed and included 47 Caucasians, one African–American and seven Asian subjects. 3.1. Clinical pain ratings
2.5. Scale preferences All participants were asked at the end of the psychophysical experiments to rank order the three scales according to the following questions: Which of the three scales did you prefer most for rating of your pain sensations? Please rank order your choices from most to least preferred. If you cannot make up your mind, use ‘‘can’t decide”. 2.6. Tender point testing All subjects underwent tender point testing as defined by the American College of Rheumatology (ACR) Criteria [26]. For this purpose nine paired tender points and two control points (at the center of the right forearm and the right thumbnail) were assessed by a trained investigator using a Fischer Dolorimeter (Pain Diagnostics, Great Neck, NY). The rubber tip of the Dolorimeter was 1 cm in diameter. The Dolorimeter was placed on the examination site, and pressure was gradually increased by 1 kg/s. The subjects were instructed to report when the sensation at the examination site changed from pressure to pain. Pressure testing was stopped at that moment and the result recorded as positive (1) if maximal pressure was 64 kg. If no pain was elicited at P4 kg the test result was recorded as negative (0). 2.7. Statistical analysis Statistical comparisons utilized SPSS 15.0 software (SPSS, Inc., Chicago, IL). Because of our interest in the discriminability of the scales for 0.5 and 1 °C steps two separate sets of mixed model ANOVAs were performed. The significance level was set at .05. Scales (3) and temperature (5) were the within subjects’ factors, and diagnosis (2) or sex (2) were the between subjects’ factor. Simple contrasts were used to decompose significant interactions effects. To test differences in scale preferences reported by the study subjects, z-tests for proportions were conducted comparing all scales. 3. Results We recruited 39 healthy subjects (females: 26; males 13) [mean age (SD): 35.3 (13.3) years] using advertisements posted throughout the University of Florida, Gainesville and 16 female FM patients [52.0 (12.0) years] from the local community and FM support groups. The chronic pain patients fulfilled the 1990 American College of Rheumatology Criteria for FM
Mean (SD) clinical pain rating [mean (SD); mechanical VAS scale: 0–10] of NC and FM subjects was 0.6 (0.6) VAS units and 3.6 (3.2) VAS units, respectively. An independent t-test indicated significantly higher pain ratings of FM subjects compared to NC (t(57) = 6.4; p < .001). There was no significant difference in clinical pain ratings between NC-F and NC-M (p > .05). 3.2. Comparisons of scaling properties of VAS, VAS-N, and NUM Statistical comparisons between ratings of ascending, descending, and random heat stimuli showed a significant difference between random and both other series (F(2, 74) = 5.2; p = .008). Study subjects rated random stimuli lower than either ascending or descending heat pulses. Ratings of ascending and descending series were not statistically different (p > .05). Because the statistical analysis of individual pain ratings during ascending, descending, and random series showed systematic differences (see Section 3.2), average ratings for each stimulus across the three scales were computed to achieve a reliable estimate. 3.2.1. Time spent to familiarize subjects with scale usage The total time spent to instruct each participant on the pain scaling methods used for the experiments was 80 s. Time spent to familiarize the subjects with (a) rating of pain intensity according to standardized instructions [17] was 30 s; (b) how to use of the electronic VAS: 15 s; (c) how to use of the electronic VAS-N: 20 s; (d) how to use of the electronic NUM scale: 15 s. All subjects reported to be comfortable with the three pain scales and were able to complete all pain scaling procedures. 3.2.2. Experimental pain ratings of heat stimuli using 1 °C increments 3.2.2.1. Pain ratings of 45–49 °C stimuli by NC-F and NC-M subjects using VAS, VAS-N or NUM scale. For all three scales, experimental pain ratings of NC-F and NC-M subjects increased monotonically for stimuli between 45 and 49 °C (see Fig. 3). A mixed model ANOVA with scales (3) and stimulus temperature (5) as within and sex (2) as between subjects’ factors showed a significant effect for temperature (F(4, 148) = 91.4; p < .001) and scales (F(2, 74) = 3.5; p = .034), without significant interaction effects of scale temperature (p > .05), indicating that all scales registered similar rate of increases in pain associated with increasing stimulus
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45 C
46 C
p < .001
47 C
48 C
49 C
p < .02
70
p < .04
p < .02
p < .01
60 ns
ns
Pain Ratings (0-100)
50 40 30 20 10 0 F
M
FM
F
VAS
M VAS -N
FM
F
M
FM
NUM
Fig. 3. Average (SE) experimental pain ratings (45–49 °C heat pulse trains; 1 °C steps) by NC-F, NC-M, and FM subjects using VAS, VAS-N, and NUM scales. All scales were able to discriminate pain sensations related to 1 °C increases in temperature regardless of diagnostic group (p < .001). For all scales except NUM, pain ratings did not statistically differ between NC-F and NC-M (p > .05). Using NUM, NC-F provided higher experimental pain ratings than NC-M (p < .01). FM subjects rated heat pulses significantly more painful than NC, regardless of scale usage (p < .04– p < .001).
intensity. There was also no sex difference noted for these monotonic increases of pain ratings (F(1, 37) = .8; p > .05). Use of simple contrasts indicated that pain ratings of subjects were higher using the NUM scale than the other two scales (F(1, 37) = 5.2; p = .03). In addition, the analysis showed a significant scale sex interaction effect (F(2, 74) = 5.6; p = .005). Further analysis revealed that NC-F rated experimental pain higher than NC-M using the NUM scale (F(1, 37) = 9.0; p = .005) but not while using the VAS or VAS-N (all p > .05). Thus, all three pain scales showed similar increases in pain ratings related to 1 °C steps in stimulus temperatures. The NUM scale, however, provided higher ratings compared to VAS and VAS-N, in particular for female subjects. The mean differences in NC subjects’ experimental pain ratings across the three scales are shown in Table 1. Most of the rating differences across scales were small, except for VAS vs Table 1 Differences scores of experimental pain ratings across three scales using the grand average of all stimuli DVAS NC-F (SD) FM-F (SD) All females (SD) NC-M (SD)
VAS-N
DVAS
NUM
DVAS-N
NUM
1.31 (5.1) 1.10 (7.4) 0.45 (6.0)
7.75 (12.1) 4.39 (6.3) 6.41 (10.3)
9.06 (14.0) 3.28 (7.2) 6.86 (12.1)
4.49 (10.8)
3.45 (10.3)
1.04 (7.8)
VAS, visual analogue scale (0–100); VAS-N, VAS with number box (0–100); NUM, numerical scale (number box) (0–100).
NUM and VAS-N vs NUM. Here the mean (SD) difference of NC-F pain ratings was 7.5 (12.1) and 9.06 (14.0) exceeding NC-M by a factor of more than two. 3.2.2.2. Pain ratings of 45–49 °C stimuli by NC-F and female FM subjects using VAS, VAS-N or NUM scale. Similar to experimental pain ratings of NC-M and NC-F subjects the ratings of FM subjects increased monotonically with increasing stimulus temperatures (see Fig. 3). Data from NC-F and female FM subjects were compared using a mixed model ANOVA with scales (3) and stimulus temperature (5) as within and diagnosis (2) as between subjects’ factors. This analysis showed main effects for scale (F(2, 80) = 10.2; p < .001), temperature (F(4, 180) = 126.3; p < .001), and diagnosis (F(1, 40) = 8.9; p = .005). There were significant interaction effects for temperature diagnosis (F(4, 160) = 3.0; p = .02) and scale temperature (F(8, 320) = 3.5; p = .001) noted. The interaction between scale and diagnosis was not significant (F(2, 80) = 1.7; p > .05). To decompose the significant interaction effects, simple contrasts were used. The results showed that the NUM scale provided higher pain ratings than the VAS (F(1, 40) = 13.1; p = .001) or the VAS-N (F(1, 40) = 10.7; p = .002). In addition, FM subjects provided higher pain rating for each temperature step compared to NC (p = .02–04). These results indicate that similar to healthy controls, FM subjects’ experimental pain ratings monotonically increased with increasing stimulus temperatures, albeit at different
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rates. Additionally, the FM subjects rated experimental pain higher using the NUM scale than the VAS or VASN. The absolute difference in female subjects’ experimental pain ratings across the three scales is shown in Table 1. Thus, independent of diagnosis, all female subjects could separately distinguish the pain associated with step-wise increases in temperature and they seemed to similarly use the three pain scales.
pared to NC-M, NC-F provided higher pain ratings using the NUM scale than using the VAS (F(1, 37) = 7.7; p = .009) or VAS-N (F(1, 37) = 4.7; p = .04). Using only NUM ratings, a mixed model ANOVA of temperatures (3) and sex (2) showed a main effect for sex (F(1, 37) = 80.0; p = .04), indicating that NC-F rated experimental pain higher than NC-M using this scale.
3.2.3. Experimental pain ratings of heat stimuli using 0.5 °C increments To evaluate the sensitivity of the three scales to change, pain rating associated with small increments in temperature (0.5 °C steps) were obtained from NC-F and NC-M as well as female FM subjects.
3.2.3.2. Pain ratings of 47–47.5–48 °C stimuli by NC-F and female FM subjects using VAS, VAS-N or NUM scale. Similar to NC participants, female FM subjects reported increasing pain intensity during 0.5 °C temperature increments between 47 and 48 °C (see Fig. 4). A mixed model ANOVA was used to analyze the subjects’ pain ratings with scale (3) and temperature (3) as within and group (2) as between subjects’ factors. There were significant main effects for scale (F(2, 80) = 6.3; p = .003), temperature (F(2, 80) = 43.4; p < .001), and group (F(1, 40) = 4.5; p = .04) noted. However, there were no significant interaction effects present for scale group, temperature group, or scale temperature (all p > .05). This lack of interactions indicates that small temperature increments resulted in similar rating increases in all groups and for all scales. The main effect for temperature indicates that the slopes for both stimulus increments were non-zero, an indication that participants could discriminate between 0.5° and 1° increments. Use of simple contrasts showed that (a) the NUM scale provided higher pain ratings than the VAS (F(1, 40) = 6.3; p = .02) or the VAS-N (F(1, 40) = 8.3; p = .006); (b) VAS or VAS-N ratings did not statistically differ within either group of subjects (p > .05); (c) all scales could distinguish pain ratings
3.2.3.1. Pain ratings of 47–47.5–48 °C stimuli by NC-F and NC-M subjects using VAS, VAS-N or NUM scale. The results indicate that for both groups all three scales provided monotonic increases of pain ratings with increasing stimulus temperature using 0.5 °C steps (see Fig. 4). A mixed model ANOVA was used to analyze the data with scale (3) and temperature (3) as within and sex (2) as between subjects’ factors. This analysis showed main effects for temperature (F(2, 74) = 36.4; p < .001) but not for scale (p > .05) or sex (p > .05). There was, however, a significant scale sex interaction effect (F(2, 74) = 5.1; p = .008) noted. Interactions of temperature sex (p > .05) or scale temperature (p > .05) were not significant. The lack of significant temperature scale interaction effect indicates that every scale could distinguish pain ratings related to 0.5 °C increments. Simple contrasts were used to decompose the scale sex interaction effect showing that com-
47 C
70
47.5 C
p < .001
p < .04
p < .02
60 Pain Ratings (0-100)
48 C
50
p < .02
p = .04 ns
ns
40 30 20 10 0 F
M VAS
FM
F
M V A S -N
FM
F
M
FM
NUM
Fig. 4. Average (SE) experimental pain ratings of NC-F, NC-M, and FM subjects during 47–48 °C heat pulse trains (0.5 °C steps) using VAS, VASN, and NUM scales. All scales were able to discriminate pain sensations related to 0.5 °C increases in temperature regardless of diagnostic group (p < .001). Except NUM (p = .42), pain ratings did not statistically differ between NC-F and NC-M (p > .05). Using NUM, NC-F provided higher experimental pain ratings than NC-M using either the VAS or VAS-N (p < .01). FM subjects rated heat pulses significantly more painful than NC, regardless of scale usage (p < .04–p < .001).
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related to small increments in heat stimuli (F(1, 40) = 17.9; p < .001). A follow-up analysis using t-tests to compare adjacent stimulus pairs showed that 55 of 59 adjacent pairs were highly statistically different, corroborating the ANOVA results. The vast majority (at both 0.5° and 1°) were highly significant (p < .001). 3.3. Scale preferences Most study subjects were able to rank order the three pain scales according to their personal preferences. Only 4 NC (10.3%) and 1 FM (6.3%) subject could not provide a scale preference. The VAS-N was the most preferred pain scale by 43.6% of NC and 50% of FM subjects followed by the VAS over all other scales (see Table 2). The electronic NUM was by far the least preferred scale. To test the significance of these differences we conducted z-tests for proportions comparing each VAS scale (VAS, VAS-N) to NUM and combined VAS (VAS = VAS-N) to NUM. The proportion of subjects preferring the VAS to NUM was significant at the 90% confidence interval. The proportion preferring VAS-N to NUM was significant at the 95% confidence interval and the combined VAS vs. NUM was significant at the 99% confidence interval. These analyses seem to clearly indicate a preference for the VAS scales over NUM scales. The rank order of the three pain scales was the same for NC and FM subjects (1 = VAS-N; 2 = VAS; 3 = NUM). 4. Discussion The characteristics of three types of electronic pain scales (VAS, VAS-N, and NUM) were tested in NC as well as in FM patients. Comparisons of measurement show that all scales were capable of discriminating experimental pain ratings related to 0.5 and 1 °C temperature steps within the nociceptive range. In addition, the pain ratings of NC-M, NC-F, and female FM patients followed very similar monotonic functions, thereby demonstrating the generality of results across different demographic groups. As expected, FM patients rated identical heat stimuli higher on all scales in comparison to NC subjects, demonstrating the capacity of these scales to detect group differences in pain sensitivity known to exist across FM and NC groups [19,23]. However, NC-F subjects gave higher NUM ratings than NCM subjects, but not higher VAS or VAS-N ratings to the Table 2 Scale preferences of study participants Group (n)
VAS (%)
VAS-N (%)
NUM (%)
Cannot decide (%)
NC (39) FM (16)
14 (36.0) 6 (37.4)
17 (43.6) 8 (50.0)
4 (10.2) 1 (6.3)
4 (10.2) 1 (6.3)
n, number of subjects; VAS, visual analogue scale; VAS-N, VAS with number box; NUM, numerical scale (number box).
range of noxious temperatures presented. Finally, after brief instructions, all three groups (total of 55 study subjects) found the scales easy to use, though most participants preferred the VAS-N or VAS over the NUM scale. In an exploratory analysis random heat stimuli were found to result in lower pain ratings than ascending or descending heat pulses. Because our study was not designed to explore this very interesting finding in more detail, future investigations will be necessary. To obtain a representative sample for our analysis, we combined the ratings of the three different heat pulse series. 4.1. Scale similarities The capacity of all three scales to discriminate ratings related to small (0.5 °C) stimulus differences is surprising, especially since the NUM has been claimed to provide only 10 pain levels [9,10]. This sensitivity to detect change is probably even greater than reported here, as suggested by the highly significant differences (p < .001) and robust effects sizes (partial g2 = .3 to .6) between adjacent 0.5 °C steps. The limit of discrimination for heat induced pain is 0.2–0.3 °C [2]. Rosier et al. [22] showed that mechanical VAS (M-VAS) could easily discriminate 1 °C steps over a range of 43–49 °C. If the capacity to discriminate small differences is equal to 0.5 °C and is similar over most or all of the 0–100 U range, then these scales clearly can discriminate greater than 10 levels of pain and have greater discriminatory power than previously believed. All three scales, including the 0–100 NUM, are likely to provide better discrimination than that described for the 0–10 NUM [9]. However, the specific use of the NUM in our study, including the track-ball manipulations that subjects were required to use, may make this scale different than a typical 0–10 NUM. Regardless of the reasons for the differences, this study is the first to demonstrate the capacity of three different types of electronic pain scales to detect such small differences in pain intensity. This capacity is highly relevant for pain measurement in both experimental and clinical settings. The capacity to detect pain changes initiated by pharmaceutical or other types of therapeutic/experimental interventions is critical for a study’s assay sensitivity, particularly the detection of sometimes small but systematic effects. The electronic VAS has been previously shown to provide positively accelerating power functions for groups as well as for individual subjects [15] and these functions strongly resemble those obtained in previous studies that used either standard or mechanical VAS [17,18]. The electronic VAS has demonstrated very high test–retest reliability (r = .88 to .99) and to accurately measure heritability of cold pressor (=60%) and contact heat-evoked pain (=26%) [16]. Importantly, measurement error accounted for only 5–8% of the variance in pain ratings [15,16]. Provided that adequate instructions
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were given, individual differences in electronic VAS ratings of pain mainly reflect actual differences in experienced pain intensity, not differences in scale use. Thus, these results add to the validation of the scales used in this study. 4.2. Does adding a number box to the electronic VAS change its measurement properties? The stimulus–response functions of VAS and VAS-N were very similar in our study. Both scales were able to discriminate small (0.5 °C) temperature steps and could detect large differences in pain sensitivity between FM and NC groups, as previously shown [19]. Although we are not recommending the VAS-N over VAS our results show that adding a number box to an electronic VAS does not seem to significantly change its measurement properties. However, adding a number box may not be comparable to adding numbers to a standard VAS, because a subject can simultaneously view all the numbers on the latter type scale. The electronic VASN provides a box, wherein the numbers increase monotonically as the red bar moves from left to right. Thus, a practical implication of this study is to use a VAS-N for those patients and healthcare practitioners who are uncomfortable with pain scales devoid of numbers, since adding a number box made no significant change in their psychometric properties. Furthermore, according to the study subjects, the VAS-N was the most preferred scale. 4.3. Does use of the 0–100 NUM scale produce a scaling bias? A previous study of orofacial pain patients (21$; 2#) showed that the mechanical VAS but not 0–10 NUM had ratio scale properties [17]. Unlike VAS, the 0–10 NUM did not display a distinct zero point (see Fig. 4 and associated data in Price et al. [17]) and 0–10 NUM ratings were found to be artificially higher for both clinical and experimental pain than VAS ratings [6,17,24]. Although both scales generate monotonic stimulus response functions, the 0–10 NUM function is displaced to the left of the VAS stimulus–response curve [17]. A consequence of this displacement of the 0–10 NUM stimulus–response function is the loss of ratio scale properties. This shift also produces artificially high ratings near pain threshold, making it less likely for NUM to have a true zero point [17]. In contrast, VAS ratings for 45 °C stimuli are close to zero. Given these differences, the 0–10 NUM almost certainly contains a scaling bias and cannot provide the same information as the VAS, though their ratings may be strongly correlated. This study shows that this scaling bias occurred only for women. This may be clinically relevant for healthcare providers if they want to assess percentage
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change in pain. The lack of NUM’s ratio scale properties would certainly introduce a systematic error in such pain measurements. The results of this study further confirm and extend our previous comparison of VAS and 0–10 NUM [17]. Similar to the previous study, female subjects gave higher NUM ratings than VAS ratings (Fig. 3 and Table 1). Although these differences were small [up to 9.02 units (scales: 0–100)] they were not negligible. Importantly, almost all of these increased pain ratings were provided by female but not by male subjects (Fig. 3 and Table 1). These differences were not found between VAS and VAS-N scales. There are two possible interpretations of these findings. One is that the 0–100 NUM is the only one capable of detecting sex differences in pain sensitivity and the other is that this scale fosters a scaling bias predominantly in female subjects. We think the latter interpretation is correct for two reasons. First, VAS has been able to detect robust sex differences in pain sensitivity in human studies [21,25]. For example, Robinson et al. [21] found reliable sex effects for cold pressor pain (Cohen’s d = .4 to .7) using the electronic VAS. Sex differences vary according to experimental pain models, ranging from large effects for cold pressor tests to very small or negligible effects for heat pulses like those used in this study [4,16,25]. Second, recent studies have shown the electronic VAS to be sensitive to individual and group differences in general [15,16]. Finally, contrary to several studies that compare VAS and NUM in clinical settings wherein acute pain is measured [1,3,6,12], our study subjects strongly preferred VAS or VAS-N to NUM (Table 2) and all the 55 subjects were able to use these scales after brief standardized instructions. Because the ratings were registered automatically, these results were free of scoring errors. Until now the user-friendliness of VAS was controversial among researchers and clinicians. The reluctance of some healthcare providers and patients to use the VAS in clinical practice has resulted in the widespread use of pain scales with inferior scaling properties (e.g. 0–10 NUM and Faces Scales with only two sad faces). In contrast, mechanical and electronic VAS have excellent psychometric properties and greatly reduce difficulties claimed to exist for standard VAS (higher cognitive demand and recording errors). 5. Conclusions The combination of results from this investigation and previous studies show that the electronic VAS has excellent pain measurement properties, including the capacity to measure very small differences. Adding a number box does not seem to change these properties significantly. However, a number box alone (NUM) appears to introduce a scaling bias, predominantly in female subjects (Table 1). Unlike many previous studies,
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the electronic VAS and VAS-N were easy to administer and score and most strongly preferred by subjects. Overall, the electronic VAS has psychometric properties that are strongly superior to 0–10 NUM and are likely to be better than the 0–100 NUM. Given the superior psychometric characteristics of VAS (mechanical and electronic) over NUM, these scales can be highly recommended for clinical research and practice.
Acknowledgments Supported by NIH Grants NS-38767 and AR053541. The expert technical assistance of Susann Nagel is gratefully acknowledged. None of the authors had any financial interests in the scales that were developed for this study.
References [1] Breivik EK, Bjornsson GA, Skovlund E. A comparison of pain rating scales by sampling from clinical trial data. Clin J Pain 2000;16:22–8. [2] Bushnell MC, Taylor MB, Duncan GH, Dubner R. Discrimination of innocuous and noxious thermal stimuli applied to the face in human and monkey. Somatosens Res 1983;1:119–29. [3] Ekblom A, Hansson P. Pain intensity measurements in patients with acute pain receiving afferent stimulation. J Neurol Neurosurg Psychiatry 1988;51:481–6. [4] Fillingim RB, Edwards RR, Powell T. The relationship of sex and clinical pain to experimental pain responses. Pain 1999;83:419–25. [5] Gerson GR, Gerson JN. Assessment of pain using a computer. Anaesthesia 1980;35:815–7. [6] Jensen MP. The validity and reliability of pain measures for use in clinical trials in adults: review paper written for the Intiative on Methods, Measurements, and Pain Assessment in Clinical Trials (IMMPACT) meeting, April 12–13 (2003). IMMPACT-II; 2003. [7] Jensen MP. The validity and reliability of pain measures in adults with cancer. J Pain 2003;4:2–21. [8] Jensen MP, Karoly P. Self-report scales and procedures for measuring pain in adults. In: Turk D, Melzack R, editors. Handbook of pain assessment. New York: Guilford Press; 2001. p. 15–34. [9] Jensen MP, Turner JA, Romano JM. What is the maximum number of levels needed in pain intensity measurement? Pain 1994;58:387–92.
[10] Jensen MP, Turner JA, Romano JM, Fisher LD. Comparative reliability and validity of chronic pain intensity measures. Pain 1999;83:157–62. [11] Kiernan BD, Dane JR, Phillips LH, Price DD. Hypnotic analgesia reduces R-III nociceptive reflex: further evidence concerning the multifactorial nature of hypnotic analgesia. Pain 1995;60:39–47. [12] Kremer E, Atkinson JH, Ignelzi RJ. Measurement of pain: patient preference does not confound pain measurement. Pain 1981;10:241–8. [13] Myles PS, Troedel S, Boquest M, Reeves M. The pain visual analog scale: is it linear or nonlinear? Anesth Analg 1999;89:1517–20. [14] Myles PS, Urquhart N. The linearity of the visual analogue scale in patients with severe acute pain. Anaesth Intensive Care 2005;33:54–8. [15] Nielsen CS, Price DD, Vassend O, Stubhaug A, Harris JR. Characterizing individual differences in heat-pain sensitivity. Pain 2005;119:65–74. [16] Nielsen CS, Stubhaug A, Price DD, Vassend O, Czajkowski N, Harris JR. Individual differences in pain sensitivity: genetic and environmental contributions. Pain 2008;136:21–9. [17] Price DD, Bush FM, Long S, Harkins SW. A comparison of pain measurement characteristics of mechanical visual analogue and simple numerical rating scales. Pain 1994;56:217–26. [18] Price DD, McGrath PA, Rafii A, Buckingham B. The validation of visual analogue scales as ratio scale measures for chronic and experimental pain. Pain 1983;17:45–56. [19] Price DD, Staud R, Robinson ME, Mauderli AP, Cannon RL, Vierck CJ. Enhanced temporal summation of second pain and its central modulation in fibromyalgia patients. Pain 2002;99: 49–59. [20] Quiton RL, Greenspan JD. Across- and within-session variability of ratings of painful contact heat stimuli. Pain 2008;137:245–56. [21] Robinson ME, Gagnon CM, Riley JL, Price DD. Altering gender role expectations: effects on pain tolerance, pain threshold, and pain ratings. J Pain 2003;4:284–8. [22] Rosier EM, Iadarola MJ, Coghill RC. Reproducibility of pain measurement and pain perception. Pain 2002;98:205–16. [23] Staud R, Koo E, Robinson ME, Price DD. Spatial summation of mechanically evoked muscle pain and painful after sensations in normal subject and fibromyalgia patients. Pain 2007;130:177–87. [24] Wallenstein SL, Heidrich III G, Kaiko R, Houde RW. Clinical evaluation of mild analgesics: the measurement of clinical pain. Br J Clin Pharmacol 1980;10:319S–27S. [25] Wise EA, Price DD, Myers CD, Heft MW, Robinson ME. Gender role expectations of pain: relationship to experimental pain perception. Pain 2002;96:335–42. [26] Wolfe F, Smythe HA, Yunus MB, Bennett RM, Bombardier C, Goldenberg DL. The American College of Rheumatology 1990 Criteria for the classification of fibromyalgia. Report of the multicenter criteria committee. Arthritis Rheum 1990;33:160–72.