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An evaluation of length and end-phrase of visual analogue scales in dental pain
Pain, 21 (1985) 177-185
177
Elsevier PA1 00717
An Evaluation of Length and End-phrase of Visual Analogue Scales in Dental Pain Robin A. Seymour *, Judy M. Simpson **, J. Ed Charlton *** and Michael E. Phillips * * The Dental
School, University of ~ewe~tie upon Tune, ** Department of Medical Statisticsz university of Newcastle upon Tyne and *** Departmenf of Anaesfhesia and Pain Relief Clinic, Royal Victoria Infirmary, Newcastle upon Tyne (U.K..)
(Received 15 June 1984, accepted 28 September 1984)
Summa~
Visual analogue scales (VAS) of different lengths (5, 10, 15 and 20 cm) and with different end-phrases (troublesome, miserable, intense, unbearable and worst pain imaginable) were used to record pain in 50 male and 50 female patients with pulpitis or pericoronitis. All 100 patients successfully completed the questionnaire. High correlation was found between the scores on all the scales. Scales of length 10 or 15 cm had the smallest meas~ement error. The scale with the end-phrase ‘worst pain imaginable’ was found to be the best choice for comparing present pain or worst pain between different groups. Using this scale no significant difference was found between the scores of males and females or between those of patients with pulpitis and pericoronitis. This study suggests the use of 10 cm visual analogue scales with the end-phrase ‘worst pain imaginable’ as being the most suitable for measuring dental pain.
introduction
Visual analogue scales are used extensively to measure clinical pain or pain relief in analgesic assays or in experimental pain. The scale is usually a 10 cm line with clearly defined boundaries. The patients or subjects are asked to place a line vertically through the scale to represent their pain. Freyd [6] suggested that the following points be considered when preparing such a scale: (1) The sensation or response to be observed should be defined. 0304-3959/85/$03.30 0 1985 Elsevier Science Publishers B.V. (Biomedical Division)
178
(2) The extremes of the sensations or responses should be decided. The endphrases should not be so extremely worded as never to be employed. It is not necessary to alternate the favourable extremes of the line. (3) The descriptive terms should be short, readily understood expressions in common use. (4) The median of the sensations or responses should be in the centre of the line. (5) Numbers should not be superimposed on a visual analogue scale because certain numbers are preferred and interfere with the distribution of the results. (6) The line should be of a length that may be grasped as a unit (10 cm is convenient) and should have definite boundaries. (7) The scale should be introduced with appropriate questions standardised before the experiment. There have been many studies [4,7,8,13,14,16] which have reported on the reliability and sensitivity of visual analogue scales as a method of recording pain. A recent report [15] compared the sensitivity of visual analogue scales with the McGill Pain Questionnaire [ll] in a sample of patients (50 males and 50 females) suffering from either pericoronitis or pulpitis. It showed that the categories ‘pain rating index’ (PRI) and ‘number of words chosen’ (NWC) from the McGill Pain Questionnaire gave a greater difference between the reported pain of pericoronitis and pulpitis and between men and women. The authors concluded that the PRI and NWC were more sensitive methods of recording pain in dental patients than the 10 cm visual analogue scale. The aim of the present study was to evaluate the effect of changing the length of the line or the end-phrase of the visual analogue scale in a further 100 patients suffering from either pericoronitis or pulpitis.
Methods Fifty patients suffering from pericoronitis and 50 patients with toothache of pulpal origin participated in the study. All patients were aged over 18 years and attended the Admissions Department of Newcastle Dental Hospital for treatment. In each group there were equal numbers of males and females, All patients were examined and questioned by the same clinician (M.E.P.). The questionnaires were self-administered and comprised one pair of simple descriptive scales (SDS) and 8 pairs of visual analogue scales (VAS). The first scale in each pair asked patients to record their pain at present (i.e., time of examination), and the second scale to record their pain at its worst. Each pair of scales was on a separate sheet of paper. The SDS consisted of the numbers 1-5 above the words ‘mild,’ ‘discomforting,’ ‘distressing,’ ‘horrible’ and ‘excruciating,’ respectively. Patients were asked to choose the number of the word that described their pain. The first 4 pairs of VAS were of different length (5,10,15 and 20 cm); all had the same boundaries - ‘no pain’ and ‘worst pain imaginable.’ The remaining 4 pairs of VAS were all 10 cm long but for each pair the words at the extreme boundary were different; they were, in consecutive order, ‘troublesome, ’ ‘miserable,’ ‘intense’ and ‘unbearable.’ These words were
179
chosen from the evaluative category of the McGill Pain Questionnaire, and nearly all the patients in the previous study [15] chose a word from this category. Furthermore, the 4 words were given the same rank order by different groups of people [12]. Patients were asked to place a vertical mark through each VAS to represent their pain. The position of the mark was subsequently measured to the nearest mm from the ‘no pain’ boundary of the scale. All analyses were performed separately for the recordings of present pain and worst pain. Histograms of the scores for all 8 types of VAS were plotted and descriptive statistics were calculated. Correlation coefficients were calculated between the 4 scales of different length and between the five 10 cm scales with different end-words. Spearman’s rank correlation coefficient was calculated between all the 10 cm VAS and the corresponding SDS. After considering the dist~butions of the scores on all 8 scales, the best scale was selected as the one with fewest observations at the extremes of the line. Using appropriate transformations of the scores from this scale, two-way analysis of variance was performed to test for a difference between the sexes or between pulpitis and pericoronitis. To compare scales of different length, we must assume that they are all measuring the same thing, with more or less precision. We postulate that there is some underlying ‘true’ pain which patients record in the VAS. This gives model 1: VAS score = constant X ‘true’ pain + error. If we also assume that multiplying the length of the line by any number k results in a direct scaling up of the VAS scores, we have model 2: VAS score for scale B = k X (VAS score for scale A) = k X constant X ‘true’ pain + k X error where scale B is k times the length of scale A. Models 1 and 2 were fitted to the data for the 4 different line lengths separately for present pain and worst pain. Both models were fitted using maximum likelihood methods and the goodness of fit was tested using likelihood ratio tests.
Results End-phrase
The histograms of the pain scores for the 10 cm scales with different end-phrases are shown in Fig. 1, for present pain and worst pain. As the end-word becomes more extreme, the scores tend to decrease, so the mass of the histogram shifts to the left. This is demonstrated in Table I, which gives the median values and numbers of observations at the extremes. The ordering of the medians is consistent with the rank order given by Melzack and Torgerson [12]. The phrase ‘worst pain imaginable’
180
seems to fall between ‘miserable’ and ‘intense,’ although it does not produce a comparable J-shaped distribution. The correlation coefficients for pairs of 10 cm scales are given in Table II. The correlations are all high, although those for worst pain are nearly all lower than the corresponding values for present pain. In order to compare different groups (males vs. females, pericoronitis vs. pulpitis), we need a scale which produces a good spread of results, with few scores at either extreme. If the distribution is slightly skewed, the arcsin transformation, which is used to stabilize the variance, may rectify this; but if many observations are clustered at a boundary, no transformation can separate them, so the scale will be
Fig. 1. Histograms of scores (mm) on VAS with different end-words for (a) pain at present, (b) pain at its worst. The frequency scale gives the number of patients recording scores in 10 mm ranges.
TABLE
I
MEDIANS
AND
EXTREME
VALUES
(mm) FOR VAS WITH
DIFFERENT
END-PHRASES
fw End-phrase
Median
Troublesome Miserable Worst imaginable Intense Unbearable
Worst pain
Present pain
50 37 25 18 15
Frequency of extreme values 0
100
13 13 14 17 18
5 9 0 3 2
Median
90 81 64 65 58
Frequency of extreme values 0
100
1 1 1 1 2
23 19 5 10 9
(n =
181
less sensitive. Inspection of Fig. 1 and Table I using these criteria leads to choosing the scale with end-phrase ‘worst pain imaginable’ for worst pain, and either ‘worst pain imaginable’ or ‘troublesome pain’ for pain at present. However, the interpretation of ‘troublesome’ appears to vary widely between patients, as Fig. 2a shows: 49 patients described their pain as ‘discomforting’ on the SDS and this pain was scored as anyt~ng from 1 to 100 mm on the VAS with ‘troublesome pain’ as its end-phrase. There is less variation in the corresponding scores from the VAS with end-phrase ‘worst pain imaginable,’ as shown in Fig. 2b. For this VAS the rank correlation with the SDS value is higher (0.78 compared with 0.67). Hence the VAS with the end-phrase ‘worst pain imaginable’ appears to be the best choice for comparing groups, whether for present or worst pain. Using the pain scores on these scales, two-way analysis of variance showed no significant difference between male and female scores or between pulpitis and pericoronitis. Applying the arcsin transformation produced similar results. Line length
The histograms for the scales of length 5, 15 and 20 cm with the end-phrase 10 cm scale. Between pairs of scales of different length the correlations were all very high, ranging from 0.90 to 0.94 for present pain and from 0.86 to 0.95 for worst pain. The means and standard deviations for the 4 scales of different length are given in Table III. ‘worst pain imaginable’ were very similar to those for the corresponding
TABLE
II
CORRELATION Coefficients
MATRIX
in the upper
FOR ALL 10 cm SCALES
right corner
are for pain at present;
Troublesome
Miserable
Troublesome Miserable Worst imaginable Intense Unbearable
TABLE MEANS
0.86
those in the lower ieft are for w&t
Worst imaginable
Intense
Unbearable
0.73
0.74 0.81 0.83
0.68 0.74 0.76 0.91
0.78
0.79 0.61 0.63 0.56
0.73 0.69 0.72
0.83 0.78
0.86
III (AND
STANDARD
DEVIATIONS)
FOR VAS OF DIFFERENT
LENGTH
Length (mm)
Present pain
Worst pain
50 100 150 200
13.6 29.8 41.2 56.4
29.6 60.4 87.5 116.4
(12.0) (24.1) (34.8) (46.1)
(12.5) (23.8) (33.6) (44.7)
(n = 100)
pain.
182 TROUBLESOME ‘r=O78
Var Score IOC
.
Y
:
: ”
!
.
! .
EC
:
.
.
. .
.
P t
. .
60
l
.
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:
.
; .
. .
”
40
.
: .
20
: .
ii
.
&
: ”
.
. ”
0
: .
a
_I.
1
2
5
4
3 S.D.S. Score (0) WORST
PAIN
IMAGINABLE
‘rr0.78 Var Score 100
t . 80
t
(r
. &
.
60
;
. . 40
.
20
C S.D.S. lb)
Score
183
For present pain, the concept of an underlying ‘true’ pain fitted the data well (likelihood ratio test Xi = 3.41; P c 0.10). However, the data were not consistent with the hypothesis that increasing the line length simply results in a scaling up of the score (Xi = 61.1; P c 0.001). Lines of length 10 and 15 cm appear to measure ‘true’ pain best (for both, scaled variance = 0.06) while a length of 5 cm gives the greatest scaled variance (0.13). For worst pain, the data were not consistent with the ‘true’ pain model (Xi = 27.1; P < 0.001). However, comparison of the multiple correlation coefficients for the 4 line lengths showed that 10 and 15 cm lines gave the highest values of R2 (0.93) while the 5 cm line had the lowest (0.85).
Visual analogue scales have been used to measure a variety of subjective responses in both psychology [6] and education [l]. It is only comparatively recently that such scales have been used to measure pain [7]. It has been reported [16] that the visual analogue scale has added a new dimension to the evaluation of clinical pain and analgesia. However, these scales have had limited use in evaluating dental pain. A recent study [9] reported that 11% of a chronic pain patient population could not complete their visual analogue scales. It was shown that those who failed to complete the scale were significantly older than those who succeeded. Subsequently, Littlejohns and Vere [lo] reported few difficulties with the completion of visual analogue scales, even with patients of low intelligence, provided that a standard introduction was used. In the present study, standard instructions based on those previously reported [lo] were given to the patients by the same clinician (M.E.P.) on every occasion. Every patient completed the questionnaire without any difficulty. Furthermore, the scores recorded on the scales of different length were highly correlated pairwise. The patients in the present study were considerably younger than those in the Kremer study [9] and the age difference may still be a contributing factor to the patients’ understanding of visual analogue scales. Patients are also easily able to adjust the position of their mark to indicate approximately the same proportion of the VAS when the length of the line is altered. Scores of present pain appear to reflect an underlying ‘true pain.’ However, this is not so for worst pain, perhaps because of a distorting effect of the memory on recollection of the ‘true pain.’ Increasing the length of the line does not result in a direct scaling up of the recorded score: the measurement error is least for scales of length 10 or 15 cm and greatest for those of 5 cm.
Fig. 2. Present pain scores from 10 cm VAS plotted against SDS scores, for VAS with end-phrase (a) troublesome pain, (b) worst pain imaginable. On the SDS the following words represent pain of increasing intensity: 1, mild; 2, discomforting; 3, distressing; 4, horrible; 5, excruciating.
184
In his original paper, Freyd [6] suggested that the extreme of the sensation or response to be measured should not be so worded as never to be employed. Studies which have employed the VAS have used a variety or phrases at the extreme of the scale. Examples include: unbearable [4,13], my pain could not be more severe 181, worst pain possible [16], as bad as it could be [5,9], worst pain imaginable [14], pain is as much as I can bear 131, severe pain [Z]. For this study 4 evaluative words (troublesome, miserable, intense, unbearable~ were chosen as alternatives to ‘worst pain imaginable.’ The results show that if the end-phrase is too extreme, too many patients will record a score of 0, thereby reducing the sensitivity of the scale. On the other hand, if the end-phrase is not extreme enough, scores for severe pain will cluster at 100, with a similar loss in sensitivity. Ideally we would like the scores to be normally distributed about a mean of 50 with variance such that the scores cover most of the line; then the median would also be in the centre of the line. However, this criterion is less important because skewness can be reduced by transforming the data. Based on these modifications of Freyd’s criteria, the end-phrase ‘worst pain imaginable’ gives the best distribution of results. The distributions of the VAS scores demonstrate one of the problems inherent in measuring dental pain. The ‘worst pain’ scores understandably tend to be high, but most patients reported only mild or discomforting ‘pain at present,’ and the corresponding VAS scores are low. This reflects the well-known phenomenon that dental pain tends to disappear when the patient seeks dental treatment. However, as the subjects of this study were a random sample of patients with pulpitis or pericoronitis, these distributions should be representative of those observed in general practice. It is surprising that the end-phrase ‘worst pain imaginable’ appears to be less extreme than ‘intense’ and ‘unbearable,’ on the basis of median scores. However, it does not have a clear position in Melzack and Torgerson’s list; it produces fewer extreme results than any other end-phrase and a different-shaped distribution. Using the most sensitive of the VAS employed in this study, no significant difference was found between scores recorded by males and females or between those given by patients with pulpitis and pericoronitis. These results confirm those of a previous study [15] using an identical VAS. This could be regarded as an advantage of the visual analogue scale, which therefore has wide applicability in studies of all types of pain in both sexes.
We would like to acknowledge the assistance of the late Professor J.A. Anderson, who suggested the models in this paper. References 1 Aitken, R.C.B., Measurement of feeiings using visual analogue scales, J. ray. Sot. Med., 62 (1969) 989-993.
185 2 Berry, M. and Husk&on, E.C., Treatment of rheumatoid arthritis, a trial of Meprothixol, Clin. Trial J., 4 (1972) 13-14. 3 Clarke, P.R.F. and Speak, F.G., Reliability and sensitivity in the self-assessment of well-being, Bull. Brit. psychol. Sot., 17 (1964) 55. 4 Downie, W.W., Leatham, P.A., Rhind, V.M., Wright, V., Branco, J.A. and Anderson, J.A., Studies with pain rating scales, Ann. rheum. Dis., 37 (1978) 378-381. 5 Dundee, J.W., Clinical evaluation of mild analgesics, Brit. J. clin. Pharmacol., 10, Suppl. (1980) 329-334. 6 Freyd, M., The graphic rating scale, J. educ. Psychol., 43 (1923) 83-102. 7 Husk&on, E.C., Measurement of pain, Lancet, ii (1974) 1127-1131. 8 Joyce, C.R.B., Zutshi, D.W., Hrubes, V. and Mason, R.M., Comparison of fixed interval and visual analogue scales for rating chronic pain, Europ. J. clin. Pharmacol., 8 (1975) 415-420. 9 Kremer, E., Atkinson, J.H. and Ignelzi, R.J., Measurement of pain: patient preference does not confound pain measurement, Pain, 10 (1981) 241-248. 10 Littlejohns, D.W. and Vere, D.W., The clinical measurement of analgesic drugs, Brit. J. clin. Pharmacol., 11 (1981) 319-332. 11 Melzack, R., The McGill Pain Questionnaire: major properties and scoring methods, Pain, 1 (1971) 277-299. 12 Melzack, R. and Torgerson, W.S., On the language of pain, Anesthesiology, 34 (1971) 50-59. 13 Ohnhaus, E.E. and Alder, R., Meth~oto~cat problems in the measu~ment of pain: a comparison between the verbal rating scale and the visual analogue scale, Pain, 1 (1975) 379-384. 14 Revill, S.I., Robinson, J.O., Rosen, M. and Hogp, M.J., The reliability of a tinear analogue for evaluating pain, Anaesthesia, 31 (1976) 1191-1198. 15 Seymour, R.A., Charlton, J.E. and Phillips, M.E., An evaluation of dental pain using visual analogue scales and the McGill Pain Questionnaire, J. oral max. Surg., 41 (1983) 643-648. 16 Wallenstein, S.L., Heidrich, G., Kaiko, R. and Houde, R.W., Clinical evaluation of mild analgesics: the measurement of clinical pain, Brit. J. clin. Pharmacol., 10, Suppl. (1980) 319-327.
177
Elsevier PA1 00717
An Evaluation of Length and End-phrase of Visual Analogue Scales in Dental Pain Robin A. Seymour *, Judy M. Simpson **, J. Ed Charlton *** and Michael E. Phillips * * The Dental
School, University of ~ewe~tie upon Tune, ** Department of Medical Statisticsz university of Newcastle upon Tyne and *** Departmenf of Anaesfhesia and Pain Relief Clinic, Royal Victoria Infirmary, Newcastle upon Tyne (U.K..)
(Received 15 June 1984, accepted 28 September 1984)
Summa~
Visual analogue scales (VAS) of different lengths (5, 10, 15 and 20 cm) and with different end-phrases (troublesome, miserable, intense, unbearable and worst pain imaginable) were used to record pain in 50 male and 50 female patients with pulpitis or pericoronitis. All 100 patients successfully completed the questionnaire. High correlation was found between the scores on all the scales. Scales of length 10 or 15 cm had the smallest meas~ement error. The scale with the end-phrase ‘worst pain imaginable’ was found to be the best choice for comparing present pain or worst pain between different groups. Using this scale no significant difference was found between the scores of males and females or between those of patients with pulpitis and pericoronitis. This study suggests the use of 10 cm visual analogue scales with the end-phrase ‘worst pain imaginable’ as being the most suitable for measuring dental pain.
introduction
Visual analogue scales are used extensively to measure clinical pain or pain relief in analgesic assays or in experimental pain. The scale is usually a 10 cm line with clearly defined boundaries. The patients or subjects are asked to place a line vertically through the scale to represent their pain. Freyd [6] suggested that the following points be considered when preparing such a scale: (1) The sensation or response to be observed should be defined. 0304-3959/85/$03.30 0 1985 Elsevier Science Publishers B.V. (Biomedical Division)
178
(2) The extremes of the sensations or responses should be decided. The endphrases should not be so extremely worded as never to be employed. It is not necessary to alternate the favourable extremes of the line. (3) The descriptive terms should be short, readily understood expressions in common use. (4) The median of the sensations or responses should be in the centre of the line. (5) Numbers should not be superimposed on a visual analogue scale because certain numbers are preferred and interfere with the distribution of the results. (6) The line should be of a length that may be grasped as a unit (10 cm is convenient) and should have definite boundaries. (7) The scale should be introduced with appropriate questions standardised before the experiment. There have been many studies [4,7,8,13,14,16] which have reported on the reliability and sensitivity of visual analogue scales as a method of recording pain. A recent report [15] compared the sensitivity of visual analogue scales with the McGill Pain Questionnaire [ll] in a sample of patients (50 males and 50 females) suffering from either pericoronitis or pulpitis. It showed that the categories ‘pain rating index’ (PRI) and ‘number of words chosen’ (NWC) from the McGill Pain Questionnaire gave a greater difference between the reported pain of pericoronitis and pulpitis and between men and women. The authors concluded that the PRI and NWC were more sensitive methods of recording pain in dental patients than the 10 cm visual analogue scale. The aim of the present study was to evaluate the effect of changing the length of the line or the end-phrase of the visual analogue scale in a further 100 patients suffering from either pericoronitis or pulpitis.
Methods Fifty patients suffering from pericoronitis and 50 patients with toothache of pulpal origin participated in the study. All patients were aged over 18 years and attended the Admissions Department of Newcastle Dental Hospital for treatment. In each group there were equal numbers of males and females, All patients were examined and questioned by the same clinician (M.E.P.). The questionnaires were self-administered and comprised one pair of simple descriptive scales (SDS) and 8 pairs of visual analogue scales (VAS). The first scale in each pair asked patients to record their pain at present (i.e., time of examination), and the second scale to record their pain at its worst. Each pair of scales was on a separate sheet of paper. The SDS consisted of the numbers 1-5 above the words ‘mild,’ ‘discomforting,’ ‘distressing,’ ‘horrible’ and ‘excruciating,’ respectively. Patients were asked to choose the number of the word that described their pain. The first 4 pairs of VAS were of different length (5,10,15 and 20 cm); all had the same boundaries - ‘no pain’ and ‘worst pain imaginable.’ The remaining 4 pairs of VAS were all 10 cm long but for each pair the words at the extreme boundary were different; they were, in consecutive order, ‘troublesome, ’ ‘miserable,’ ‘intense’ and ‘unbearable.’ These words were
179
chosen from the evaluative category of the McGill Pain Questionnaire, and nearly all the patients in the previous study [15] chose a word from this category. Furthermore, the 4 words were given the same rank order by different groups of people [12]. Patients were asked to place a vertical mark through each VAS to represent their pain. The position of the mark was subsequently measured to the nearest mm from the ‘no pain’ boundary of the scale. All analyses were performed separately for the recordings of present pain and worst pain. Histograms of the scores for all 8 types of VAS were plotted and descriptive statistics were calculated. Correlation coefficients were calculated between the 4 scales of different length and between the five 10 cm scales with different end-words. Spearman’s rank correlation coefficient was calculated between all the 10 cm VAS and the corresponding SDS. After considering the dist~butions of the scores on all 8 scales, the best scale was selected as the one with fewest observations at the extremes of the line. Using appropriate transformations of the scores from this scale, two-way analysis of variance was performed to test for a difference between the sexes or between pulpitis and pericoronitis. To compare scales of different length, we must assume that they are all measuring the same thing, with more or less precision. We postulate that there is some underlying ‘true’ pain which patients record in the VAS. This gives model 1: VAS score = constant X ‘true’ pain + error. If we also assume that multiplying the length of the line by any number k results in a direct scaling up of the VAS scores, we have model 2: VAS score for scale B = k X (VAS score for scale A) = k X constant X ‘true’ pain + k X error where scale B is k times the length of scale A. Models 1 and 2 were fitted to the data for the 4 different line lengths separately for present pain and worst pain. Both models were fitted using maximum likelihood methods and the goodness of fit was tested using likelihood ratio tests.
Results End-phrase
The histograms of the pain scores for the 10 cm scales with different end-phrases are shown in Fig. 1, for present pain and worst pain. As the end-word becomes more extreme, the scores tend to decrease, so the mass of the histogram shifts to the left. This is demonstrated in Table I, which gives the median values and numbers of observations at the extremes. The ordering of the medians is consistent with the rank order given by Melzack and Torgerson [12]. The phrase ‘worst pain imaginable’
180
seems to fall between ‘miserable’ and ‘intense,’ although it does not produce a comparable J-shaped distribution. The correlation coefficients for pairs of 10 cm scales are given in Table II. The correlations are all high, although those for worst pain are nearly all lower than the corresponding values for present pain. In order to compare different groups (males vs. females, pericoronitis vs. pulpitis), we need a scale which produces a good spread of results, with few scores at either extreme. If the distribution is slightly skewed, the arcsin transformation, which is used to stabilize the variance, may rectify this; but if many observations are clustered at a boundary, no transformation can separate them, so the scale will be
Fig. 1. Histograms of scores (mm) on VAS with different end-words for (a) pain at present, (b) pain at its worst. The frequency scale gives the number of patients recording scores in 10 mm ranges.
TABLE
I
MEDIANS
AND
EXTREME
VALUES
(mm) FOR VAS WITH
DIFFERENT
END-PHRASES
fw End-phrase
Median
Troublesome Miserable Worst imaginable Intense Unbearable
Worst pain
Present pain
50 37 25 18 15
Frequency of extreme values 0
100
13 13 14 17 18
5 9 0 3 2
Median
90 81 64 65 58
Frequency of extreme values 0
100
1 1 1 1 2
23 19 5 10 9
(n =
181
less sensitive. Inspection of Fig. 1 and Table I using these criteria leads to choosing the scale with end-phrase ‘worst pain imaginable’ for worst pain, and either ‘worst pain imaginable’ or ‘troublesome pain’ for pain at present. However, the interpretation of ‘troublesome’ appears to vary widely between patients, as Fig. 2a shows: 49 patients described their pain as ‘discomforting’ on the SDS and this pain was scored as anyt~ng from 1 to 100 mm on the VAS with ‘troublesome pain’ as its end-phrase. There is less variation in the corresponding scores from the VAS with end-phrase ‘worst pain imaginable,’ as shown in Fig. 2b. For this VAS the rank correlation with the SDS value is higher (0.78 compared with 0.67). Hence the VAS with the end-phrase ‘worst pain imaginable’ appears to be the best choice for comparing groups, whether for present or worst pain. Using the pain scores on these scales, two-way analysis of variance showed no significant difference between male and female scores or between pulpitis and pericoronitis. Applying the arcsin transformation produced similar results. Line length
The histograms for the scales of length 5, 15 and 20 cm with the end-phrase 10 cm scale. Between pairs of scales of different length the correlations were all very high, ranging from 0.90 to 0.94 for present pain and from 0.86 to 0.95 for worst pain. The means and standard deviations for the 4 scales of different length are given in Table III. ‘worst pain imaginable’ were very similar to those for the corresponding
TABLE
II
CORRELATION Coefficients
MATRIX
in the upper
FOR ALL 10 cm SCALES
right corner
are for pain at present;
Troublesome
Miserable
Troublesome Miserable Worst imaginable Intense Unbearable
TABLE MEANS
0.86
those in the lower ieft are for w&t
Worst imaginable
Intense
Unbearable
0.73
0.74 0.81 0.83
0.68 0.74 0.76 0.91
0.78
0.79 0.61 0.63 0.56
0.73 0.69 0.72
0.83 0.78
0.86
III (AND
STANDARD
DEVIATIONS)
FOR VAS OF DIFFERENT
LENGTH
Length (mm)
Present pain
Worst pain
50 100 150 200
13.6 29.8 41.2 56.4
29.6 60.4 87.5 116.4
(12.0) (24.1) (34.8) (46.1)
(12.5) (23.8) (33.6) (44.7)
(n = 100)
pain.
182 TROUBLESOME ‘r=O78
Var Score IOC
.
Y
:
: ”
!
.
! .
EC
:
.
.
. .
.
P t
. .
60
l
.
”
:
.
; .
. .
”
40
.
: .
20
: .
ii
.
&
: ”
.
. ”
0
: .
a
_I.
1
2
5
4
3 S.D.S. Score (0) WORST
PAIN
IMAGINABLE
‘rr0.78 Var Score 100
t . 80
t
(r
. &
.
60
;
. . 40
.
20
C S.D.S. lb)
Score
183
For present pain, the concept of an underlying ‘true’ pain fitted the data well (likelihood ratio test Xi = 3.41; P c 0.10). However, the data were not consistent with the hypothesis that increasing the line length simply results in a scaling up of the score (Xi = 61.1; P c 0.001). Lines of length 10 and 15 cm appear to measure ‘true’ pain best (for both, scaled variance = 0.06) while a length of 5 cm gives the greatest scaled variance (0.13). For worst pain, the data were not consistent with the ‘true’ pain model (Xi = 27.1; P < 0.001). However, comparison of the multiple correlation coefficients for the 4 line lengths showed that 10 and 15 cm lines gave the highest values of R2 (0.93) while the 5 cm line had the lowest (0.85).
Visual analogue scales have been used to measure a variety of subjective responses in both psychology [6] and education [l]. It is only comparatively recently that such scales have been used to measure pain [7]. It has been reported [16] that the visual analogue scale has added a new dimension to the evaluation of clinical pain and analgesia. However, these scales have had limited use in evaluating dental pain. A recent study [9] reported that 11% of a chronic pain patient population could not complete their visual analogue scales. It was shown that those who failed to complete the scale were significantly older than those who succeeded. Subsequently, Littlejohns and Vere [lo] reported few difficulties with the completion of visual analogue scales, even with patients of low intelligence, provided that a standard introduction was used. In the present study, standard instructions based on those previously reported [lo] were given to the patients by the same clinician (M.E.P.) on every occasion. Every patient completed the questionnaire without any difficulty. Furthermore, the scores recorded on the scales of different length were highly correlated pairwise. The patients in the present study were considerably younger than those in the Kremer study [9] and the age difference may still be a contributing factor to the patients’ understanding of visual analogue scales. Patients are also easily able to adjust the position of their mark to indicate approximately the same proportion of the VAS when the length of the line is altered. Scores of present pain appear to reflect an underlying ‘true pain.’ However, this is not so for worst pain, perhaps because of a distorting effect of the memory on recollection of the ‘true pain.’ Increasing the length of the line does not result in a direct scaling up of the recorded score: the measurement error is least for scales of length 10 or 15 cm and greatest for those of 5 cm.
Fig. 2. Present pain scores from 10 cm VAS plotted against SDS scores, for VAS with end-phrase (a) troublesome pain, (b) worst pain imaginable. On the SDS the following words represent pain of increasing intensity: 1, mild; 2, discomforting; 3, distressing; 4, horrible; 5, excruciating.
184
In his original paper, Freyd [6] suggested that the extreme of the sensation or response to be measured should not be so worded as never to be employed. Studies which have employed the VAS have used a variety or phrases at the extreme of the scale. Examples include: unbearable [4,13], my pain could not be more severe 181, worst pain possible [16], as bad as it could be [5,9], worst pain imaginable [14], pain is as much as I can bear 131, severe pain [Z]. For this study 4 evaluative words (troublesome, miserable, intense, unbearable~ were chosen as alternatives to ‘worst pain imaginable.’ The results show that if the end-phrase is too extreme, too many patients will record a score of 0, thereby reducing the sensitivity of the scale. On the other hand, if the end-phrase is not extreme enough, scores for severe pain will cluster at 100, with a similar loss in sensitivity. Ideally we would like the scores to be normally distributed about a mean of 50 with variance such that the scores cover most of the line; then the median would also be in the centre of the line. However, this criterion is less important because skewness can be reduced by transforming the data. Based on these modifications of Freyd’s criteria, the end-phrase ‘worst pain imaginable’ gives the best distribution of results. The distributions of the VAS scores demonstrate one of the problems inherent in measuring dental pain. The ‘worst pain’ scores understandably tend to be high, but most patients reported only mild or discomforting ‘pain at present,’ and the corresponding VAS scores are low. This reflects the well-known phenomenon that dental pain tends to disappear when the patient seeks dental treatment. However, as the subjects of this study were a random sample of patients with pulpitis or pericoronitis, these distributions should be representative of those observed in general practice. It is surprising that the end-phrase ‘worst pain imaginable’ appears to be less extreme than ‘intense’ and ‘unbearable,’ on the basis of median scores. However, it does not have a clear position in Melzack and Torgerson’s list; it produces fewer extreme results than any other end-phrase and a different-shaped distribution. Using the most sensitive of the VAS employed in this study, no significant difference was found between scores recorded by males and females or between those given by patients with pulpitis and pericoronitis. These results confirm those of a previous study [15] using an identical VAS. This could be regarded as an advantage of the visual analogue scale, which therefore has wide applicability in studies of all types of pain in both sexes.
We would like to acknowledge the assistance of the late Professor J.A. Anderson, who suggested the models in this paper. References 1 Aitken, R.C.B., Measurement of feeiings using visual analogue scales, J. ray. Sot. Med., 62 (1969) 989-993.
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