Validity and reliability of rating scales for seated pressure discomfort

•

r, I n t e r n a t i o n a l J o u r n ~ l of

Industrial Ergonomics ELSEVIER

International Journal of Industrial Ergonomics 20 (1997) 441 461

Validity and reliability of rating scales for seated pressure discomfort Wenqi Shen*, Kenneth C. Parsons Department (?/Human Sciences, Loughborough Universi~ t~["Technology, Ashby Road, Loughborough, Leicestershire LE11 3TU, UK Received 1 December 1995; revised 30 August 1996

Abstract

To investigate seated pressure discomfort requires a valid and reliable technique to measure discomfort. The aims of this study were to test the validity and reliability of several rating scales and select the best for investigation of pressure discomfort. Six scales were tested: a category partitioning scale, the Borg CR-10 scale, the Corlett discomfort scale, an 8-point ordinal scale, a modified intensity and discomfort scale, and a 21-point ratio scale. Twelve subjects took part in the repeated measures test in two sessions, one week apart. A test seating device generated interface pressure from underneath a foam cushion. Four levels of stimulus, 60, 85, 120, and 165 mmHg, were presented to the seated mid-thigh region. Perceived pressure intensity, discomfort level due to the pressure, and overall discomfort were reported using each of the six scales. Reliability was examined by test-retest correlation, relative rating change and coefficient of variations, and validity examined by absoluteness of rating and the functional consistency. Generally, subjects were capable of reporting their sensation of pressure intensity and discomfort by using the rating scale technique. However, the accuracy of rating strongly depended upon the properties of the scale. The category partitioning scale was found to be highly reliable and most valid for rating pressure intensity and perceived discomfort. This scale was also preferred by subjects when compared with the other five scales. Properties of all the six scales were defined and summarised. Relevance to industry

The psychometric properties of the six scales provide a basis for industrial engineers and ergonomists in selecting rating instruments for product usability as well as seating comfort. The validated category partitioning scale is very sensitive and accurate. It will find wide application in comfort evaluation and in industrial benchmarking activities.

Keywords." Pressure sensation; Psychophysics; Scaling methods; Seating; Subjective judgment

1. Introduction

Pressure distribution m e a s u r e m e n t on the seato c c u p a n t interface is a p r i m a r y tool for seating * Corresponding author. Tel.: (810) 986-9760; fax: (810) 986-7292; e-mail: [email protected].

comfort evaluation (Lee et al., 1990; Z a c h a r k o w , 1988; Schneider and Ricci, 1989). If the interface pressure in any region on the interface is excessive for prolonged sitting, it will induce compression discomfort (Hertzberg, 1958, 1972; C h o w and Odell, 1978; D r u m m o n d et al., 1982; Bader, 1986). Applied research and industrial b e n c h m a r k i n g in

0169-8141/97/$17.00 Copyright :.~! 1997 Elsevier Science B.V, All rights reserved PII S01 6 9 - 8 1 4 1 ( 9 6 ) 0 0 0 6 8 - 6

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W. Shen, K.C. Parsons / International Journal of Industrial Ergonomics 20 (1997) 44l 461

seat development give much attention to pressure discomfort. Reducing pressure magnitude and uniformly distributing pressure have been included as main specifications in seat design (Congleton, 1983; Matsuoka and Hanai, 1988). However, it is not an easy task to establish the relationship between surface pressure and seated discomfort. Measurement of surface pressure is an obvious obstacle and, until recent years, no reliable and accurate pressure systems were available. Other sources of seated discomfort, such as vibration, thermal condition and posture incur formidable "ambient noise" on pressure discomfort studies. The fundamental problem is the measurement of discomfort itself. Discomfort is a generic and subjective sensation that arises when human physiological homeostasis, psychological well-being, or both, are negatively affected. Pressure-induced discomfort is a physiological aspect of human discomfort. The measurement of pressure discomfort is a psychophysical problem. Rating-scale technique is a natural and convenient approach and it has been widely used for intensity and comfort or discomfort assessment. By adopting this technique, we assume that subjects are capable of making a direct judgement of sensations, with no need for a standard contrast stimulus. However, ratings are easily biased due to shortcomings in scale structure, instruction, stimulus context, and so on (Poulton, 1982). An appropriate rating scale will help reduce the measurement error of discomfort. Unfortunately we found no literature on the scaling properties of rating scales for postural and seating discomfort. The objectives of this study were to investigate the validity and reliability of a number of rating scales and choose the best for pressure intensity and discomfort in seated human thigh area. Validity concerns whether the scale measures what it is intended to. As seating comfort is multidimensional, inappropriate scales and testing procedures may elicit ratings on some subjective trait other than comfort or discomfort. Reliability refers to the extent to which the scale could produce the same score in similar circumstances, The most valid and reliable scale will be chosen for further investigations on the psychophysical functions of pressure discomfort, in order to quantify the contribution of external pressure to overall seated discomfort.

2. Selection of scales for rating intensity and discomfort Pitrella and K/ippler (1988) summarized 14 rating-scale design principles: (1) use continuous scales rather than category scale formats; (2) use both verbal descriptors and numbers at scale points: (3) use descriptors at all major scale markings; (4) use horizontal rather than vertical scale formats; (5) either use extreme or no descriptors at end points; (6) use short, precise, and value-unloaded descriptors; (7) use empirically determined rank-ordered descriptors; (8) select and use equidistant descriptors; (9) use psychologically scaled descriptors; (10t use positive numbers only; (11) have desirable qualities increase to the right; (12) use descriptors free of evaluation demands and biases; (13) use 11 or more scale points as available descriptors permit; (14) minimize rater workload with suitable aids. For clarity, some principles such as items (7), (8), (9), and items (6) and (12), can be combined. These principles provide a detailed checklist for scale design, depending upon task requirements. In selecting rating scales for pressure discomfort. aspects pertinent to test validity and reliability must be observed. There are numerous rating scales for comfort and discomfort, or other intensities such as exertion, workload, physical stress, and pain. Some of the scales, while useful in practical situations, were not suitable for the purpose of rating pressure intensity and discomfort. Initial selection of scales for this study were based on the following considerations: 1. The scales should measure only one dimension, i.e. discomfort or pressure intensity. A bipolar comfort-discomfort scale might be appropriate for general seat evaluation, but a study of pressure-induced sensation is concerned with discomfort only. The bipolarity of a comfortdiscomfort scale may alter the construct and reduce the available rating space for discomfort on the scale, inducing ranging effect (Poulton, 1977). The general comfort rating scale by Shackel et al. (1969) was thus not included. 2. The scales should have face validity. They should appear to measure what is intended, which is discomfort or pressure intensity. Scales such as Borg's (1970) 15-point RPE scale are not

W. Shen, K.C. Parsons / International Journal of lndustrial Ergonomics 20 (1997) 441 461

included because their numbering from 6 to 20 is specifically valid for rating physical exertion or efforts in relation to heart rates. The results would be difficult to interpret if such a scale is used for rating pressure discomfort. 3. The structure of scales before anchoring should allow for paired ratings of both pressure intensity and perceived discomfort. A study by Oborne and Clarke (1975) indicated that the type of anchoring phrases, which define the construct to be measured, do not differentially affect rating reliability in any significant way, as long as each type of anchoring consistently denotes the same construct. 4. The scales should be capable of quantifying, rather than just ranking, the intensity of pressure and discomfort. Sensitivity is essential. Coarse scales may suffer from serious ceiling or range effect. Ordinal scales and 3- or 5-point interval scales are thus excluded. Several scales for discomfort and other intensities are selected for review. Corlett and Bishop (1976), Body Part DiscomJbrt Scale. The body part discomfort (BPD) scale was developed as an element of the BPD rating procedure by Corlett and Bishop (1976). During a study of the effect of posture on discomfort in spot welding, subjects were asked lo indicate, on a body map with 12 body areas, the most uncomfortable body area(s), at ¼ hour intervals throughout a 3 h working period. A 5-point scale was used, from "0 = no discomfort" to "5 = extreme discomfort". The scale however was not graphically depicted. The concept of the BPD scale was based on the work of Kirk and Sadoyama (1973), where a 5-point scale ranging from "just noticeable discomfort" to "extremely uncomfortable" was used. Pain intensities for holding arms at varied angles were reported to be proportional to the holding time, i.e. pain intensity linearly related to the duration. The scale can thus be used as a linear scale with time (Corlett, 1991}. However, whether the BPD scale validly measures discomfort at different load levels is still unknown, Corlett (in Wilson and Corlett, 1991) found that ratings deviated from linearity at high load levels, indicating a bias due to range effect. The BPD scale has been extensively used for chair and seat evaluation. There are different modi-

443

fled versions. Drury and Coury (1982) used the BPD scale to evaluate a manual handling task and a prototype chair. Rating on the scale was defined as obtaining "a response from 1 (just noticeable pain/discomfort) to 5 (intolerable pain/discomfort)". The scale was reportedly easy to use and required almost no training for the subjects. Congleton (1983) and Thomas et al. (1991) modified the BPD scale. Subjects were instructed to place an "x" on the horizontal scale of 10 cm long. The distance between "x" and the left end of the scale represented the level of discomfort. The left end of the line was assigned as 1 for "just noticeable" discomfort. If no mark was placed, a zero value was given indicating no pain/discomfort. Borg (1982), Category-Ratio Scale ( C R- I O). Since 1962, Borg (1962, 1970, 1982) has developed different numerical scales for perceived exertion (RPE) and other physical intensities. The most accepted is the RPE scale developed in 1970, which used 15 points as opposed to the previous 21-point scale (Borg, 1962). Validity and reliability of the scale in evaluating physical and mental exertion and physical ability of athletes have been extensively tested (Skinner et al., 1973; L611gen et al., 1975). Borg (1982) recently developed a 10-point category-ratio scale (CR-10), aimed at intensity ratings in wider psychophysical dimensions. It was claimed that, for rating perceived exertion, the CR-10 scale produced data that fit a power function. The greyness test indicated that the individuals had enough knowledge to use the scale (Johansson and Ljunggren, 1989). Correlations between the colour intensities and the ratings were reportedly high (r > 0.90). Gustafson-SSderman (1987) however used the CR10 scale to assess local discomfort from the back and neck-shoulder regions in building crane operators and found that most of the ratings were lower than 2. It seems that the sensitivity of the scale was low. No reports on its validity and reliability were found. Category Partitioning Scale (CP-50). This scale is used with its unique rating procedure. Heller (1980, 1985, cited from Ellermeie; and Westpha], 1991) developed this scaling procedure for rating pain intensity. Arranged vertically, the scale has a starting point, 0 = no, and 5 categories: very' slight, slight, medium, severe, very severe. Each of the

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W. Shen, K.C. Parsons / International Journal q[ lndustrial Ergonomics 20 (1997) 441 461

categories is further divided into 10 scale points. Points above 50 are provided to avoid the ceiling effect on ratings for any extreme intensity. Subjects were directed to categorize stimuli in reference to their daily experience with pain intensity rather than simply ordering experimental stimuli. This is achieved through a two-step procedure, in which subjects first have to name the category to which the stimulus "belongs", and then may fine tune using numerical subdivisions. The scale has behaved in an "absolute" way (Ellermeier and Westphal, 1991), without stimulus context bias. A test (G6bel et al., 1988) on the reliability and validity of the scale for pain rating proved that the scale is appropriate for quantifying pain intensity. Four scales were thus selected from current literature. Scale A is the CP-50 scale, Scale B was modified by Congleton (1983) from Corlett and Bishop (1976), Scale C is the 8-point discomfort scale used by Yu et al. (1988), Yu and Keyserling (1989), and Scale D is the Borg CR-10 scale. Scale E was used by the authors in an unpublished study on sitting discomfort, and introduced here to compare with Scale B, as they have different left end anchors. Suggested by a pilot experiment on psychophysical functions of pressure sensation, we also constructed Scale F to possess ratio properties for rating intensity and discomfort. A complete set of selected scales with instructions is provided in the appendix.

3. Method

3.1. Subjects" Twelve subjects, six male and six female, were recruited as paid volunteers for the two-session experiment. The anthropometric statistics are listed in Table 1. No subjects had previous experience in scaling pressure sensations. All subjects filled out a consent form. Subjects wore boxer type shorts for the test in order to apply the pressure stimuli to the bare mid-thigh.

3.2. Apparatus and stimuli All test sessions were carried out in a controlled laboratory. The room temperature was 23 + 2°C

Table 1 Subject characteristics tn = 12)

Age Weight (kg) Height (mm) Popliteal height (ram) Buttock-popliteal length (mm) Sitting height (ram)

Mean

S.D.

Min.

35.5 68.9 1703.7 454.1 478.9 903.6

14.44 18 10.20 52.6 82.62 1546 26.37 411 21.81 446 45.63 809

Max. 58 83 1823 488 510 973

and the relative humidity was between 48% and 60%. Stimulus pressure was presented by means of a test seat. The seat cushion consisted of a wooden base and a piece of urethane foam having a density of 50 kg/m 3 and a stiffness of 90-105 N (Hairlok Ltd., UK). The wooden base consisted of slices of different width. On the stimulus slice there was a hole to accommodate a circular block of 65 mm in diameter. This stimulus block, with a flat top surface, was slowly pushed up or lowered from the wooden base level by a lead screw, which was in turn coupled to a flexible driving shaft and driven by an electric screw driver (Black and Decker, 9019 Type 2). To apply pressure stimuli on the thigh when a subject was seated, the stimulus block was driven up from underneath the foam. By arranging the sequence of base slices, the stimulus slice could be placed at any anterior-posterior location. By moving the stimulus slice from one side to the other, the stimulus block could be at any lateralmedial location. Therefore, pressure stimulus could be applied on any seated body region in contact with the seat cushion. Since the stimuli block had a nominal area of 3318 mm 2, the nature of the stimuli was surface pressure as opposed to a sharp localized pressure or hydrostatic pressure. The stimulus magnitude was measured on the subject-cushion interface, using a Talley pressure monitor system (TPM, Talley Group Ltd., UK). The sensor pad consisted of 96 pressure sensors and was arranged on the cushion surface. Each sensor was 20 mm in diameter, and the central distance from adjacent sensors was 28 mm. The calibrated repeatability of measurement was within + 3 mmHg. The stimulus area

W. Shen, K. C. Parsons / International Journal of Industrial Ergonomics 20 (1997) 441 461

was covered with 4 adjacent cells, and the largest reading among the four was taken as the stimulus magnitude. A micro-computer was connected to the T P M to download data for further analysis. The stimulus was applied at half the distance between the buttock and popliteal length under the right thigh. The stimulus block was adjusted according to individual buttock-popliteal length. Footrest height was adjusted for the subject to rest the foot while the thighs were naturally horizontal on the seat cushion. During the test, subjects were required to sit upright, without using the backrest.

3.3. Subjective judgements The rating-scale method was used. The items for paired rating included the sensations of • pressure intensity; • discomfort level due to the stimulus, and • overall discomfort on the whole seat surface. The experimenter presented the stimulus under the right thigh. When the pressure reached the required level and stabilized for 60 s, the subject reported his or her sensations of the three items by either giving verbal ratings, as for Scales A, C, D and F, or drawing backslashes on the graphic scales, as for Scales B and E. At the end of the experiment, each subject gave his or her preference of the scales by completing the last sheet in the questionnaire. Any sensations under the left thigh where no stimuli were applied, were also reported for the purpose of balance.

3.4. Experimental design The experiment consisted of two sessions. Two series of stimulus pressure levels were used, each involving three levels that were geometrically spaced. Those levels were equivalent to the range of typical interface pressures under the ischium. The high series was 85, 120, and 165 mmHg, and the low series was 60, 85, and 120mmHg. The two overlapping levels were for eliciting test-retest correlation. The twelve subjects were divided into two groups. One group received the high series of stimulus levels in the first session followed by the low series in the second session. The other group

445

Table 2 Schematic stimulus arrangements of two groups in two sessions Subject Group

Session 1 60

85

120

165

60

85

X

X

X

X

X

X

x

x

X

x

X

1 2

Session 2 120

165

X

Note: 60, 85, 120, and 160 are stimulus levels in m m t t g . ~'x'" means that the stimulus level was tested.

experienced the stimuli in the opposite order. In each session, each subject was tested with all six scales in 90 min. For each scale, each of the three stimulus levels was presented and rated twice. Therefore, 36 stimuli were tested per session. The stimulus arrangements with each scale are shown in Table 2. Four 6 x 6 Latin squares were used to arrange the scale order. The standard form of the Latin square (Fisher and Yates, 1963) was randomized to constitute a test square for the first session of the first subject group. Three additional Latin squares were obtained by permutations of the first square. Sequences of presenting the 36 stimuli were arranged using a 3 x 3 standard Latin square. The total arrangement of scale order and stimulus sequence thus reached a completely balanced design in terms of scales as well as the carry-over effects of stimuli (Table 3). To quantify any fatigue effect due to the session length, a catch-up rating was arranged at the end of each session. In order to collect subjective opinion as to which scale subjects most preferred, a questionnaire was filled out when both of the sessions finished.

3.5. Procedure Each subject participated in two test sessions of 90 min each, at an one week interval. The subject was first briefed on postural requirements, the procedure, and the scales with rating instructions. Then the subject filled out a consent form. The experimenter took anthropometric measurement and adjusted the seat settings according to body dimensions. Next, the subject sat on the test seat with standard posture: sitting symmetrically to the

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W. Shen, K.C. Parsons / International Journal of Industrial Ergonomics 20 (1997) 441- 461

central line on the seat surface; keeping the back upright and not using the backrest; keeping feet horizontal on the footrest with gauged thickness; maintaining the distance between under-knee and the front edge of the seat at about 10 mm so that the thigh-leg angle was 9 5 . Position of the stimulus block on seat cushion was adjusted relative to the measured buttock-popliteal length to ensure the center of the block at the mid-thigh. Two sample stimuli were applied on the subject's thigh for trial rating. The scale sheets were prepared in the particular order for each subject, and placed on a stand at 0.75 m in front of the subject's eye level. To present pressure, the stimulus block was driven up under the seated thigh. Pressure level was monitored through the TPM, and tuned by manually turning the screwdriver. The stimulus was presented for 60 s from the moment the pressure stimulus stabilized. The subject then reported his or her ratings in terms of pressure intensity, discomfort level due to the stimulus, and overall discomfort on the seat cushion before rising from the seat for step tests. The step tests, lasting 30 s, involved going up and down two steps, at heights of 140 and 280 mm, respectively. The rhythm was controlled by a metronome at 48 beats/min. Meanwhile, the experimenter re-adjusted the pressure monitor and the stimulus board. On finishing the step test, the subject returned to the seat for the next stimulus. The period of actual testing then started. Altogether 36 stimuli, 6 for each scale, were applied to the subject's under-thigh and rated. Each stimulus rating and step test cycle took 2 rain. The second session was conducted at the same time one week later. There was no pre-test period. When all 36 stimuli of the other series had been presented, the subject filled out the questionnaire regarding scale preferences.

4. Results Rating scores were extracted when each session finished. Ratings on the four category scales, Scale A, C, D and F, were voted by subjects. Ratings on Scale E were obtained by measuring the line length, in cm, from left most point to the marked point. Ratings on Scale B were treated differently

447

(Congleton, 1983): if it was "no sensation at all", the rating was 0; otherwise, any rating between "just noticeable" to "extreme" was scored as the sum of 0.5 cm and the actual line length, in cm. Statistical analysis including reliability test, was conducted using SPSS(R) for Windows TM (Norusis, 1993). Ratings of sensations on the left thigh were all zero or close to zero, therefore no analysis was conducted.

4.1. Reliability The reliability of the rating scales was evaluated in terms of three aspects: (1) test-retest correlation, (2) between-session change of ratings, and (3) average coefficients of variation. While the first two aspects indicate the consistency of ratings over the time interval, the third measure provides an index of internal consistency in each subject. Test-retest correlation. Test retest correlation was computed from the subject's summed ratings of two replications on each of the two common stimulus levels, 85 and 120 mmHg. In the first session, the two stimulus levels for sub-group one were extracted from the higher stimulus series of 85, 120, and 165 mmHg, and for subgroup two from the low stimulus series of 60, 85, and 120 mmHg. In the second session the range was reversed. The correlation coefficients are listed in Table 4. Scale A had the highest correlation for pressure intensity, discomfort level, and overall discomfort ratings. Scale D had high correlation for the overall discomfort rating, but not for pressure intensity and discomfort level ratings. The correlation coefficients did not differ much between stimulus levels of 85 and 120 mmHg, suggesting that the repeatability of ratings was independent of the stimulus magnitude. This property is desirable for practical applications. The highest correlation coefficients of Scale A, if converted into the intraclass correlation coefficient (ICC), is equivalent to an ICC of 0.67. According to Fleiss (1986), this shows a "good" but not "excellent" reproducibility. An excellent reproducibility should give an ICC greater than 0.75. The principal reason for this is that the test-retest was conducted on two different stimulus ranges instead of being on the same range. For relative comparison among different scales, the result is valid and Scale A is the

t~ Shen, K. C. Parsons / International Journal of Industrial Ergonomics 20 (1997) 441 461

448

Table 4 Pearson correlation coefficients of test-retest ratings on the six scales Scale

A B C D E F

Pressure intensity

Discomfort level

Overall discomfort

85

120

Avg

85

120

Avg

85

120

Avg

0.71" 0.103 0.389 0.644' 0.084 0.149

0.71 '° 0.236 0.348 0.471 0.056 0.122

0.71"" 0.169 0.368 0.558 0.07 0.136

0.365 0.26 0.214 0.547 0.374 0.315

0.692" 0.072 0.158 0.373 - 0.121 0.307

0.529 0.166 0.186 0.46 0.126 0.311

0.592" 0.413 0.334 0.762'" 0.594" 0.34

0.604" 0.245 0.228 0.658' 0.023 0.59"

0.598' 0.329 0.28l 0.71"* 0.309 0.465

Note: Computed from the s u m m e d ratings of two replications, n - 12. d.f. = 10, **p < 0.01; *p < 0.05.

Table 5 Medians (n = 12) of between-session change in ratings (in %) Scale

A B C D E F

Pressure intensity

Discomfort level

Overall discomfort

85

120

Avg

85

120

Avg

85

120

Avg

12.6 28.5 43.8 28.6 36.1 46.9

17.1 26.5 31.8 33.3 24.2 59.6

14.8 27.5 37.8 31.0 30.1 53.2

25.7 41.4 100.0 36.7 29.5 56.3

31.8 21.7 65.2 25.0 29.2 50.0

28.7 31.5 82.6 30.8 29.3 53.1

30.8 31.5 58.3 35.4 46.7 55.8

34.3 23.7 79.2 19. l 32.2 71.8

32.5 27.6 68.8 27.3 39.4 63.8

Note: The between-session change is the ratio of the average rating difference between two sessions to the mean rating in the first session.

most reliable in terms of its test-retest reproducibility. Between-session change of ratings. The betweensession change is the ratio of difference in ratings between two sessions to the rating in the first session. A smaller change indicates higher consistency in ratings. Medians of the between-session changes are presented in Table 5. For pressure intensity, Scale A exhibited an average between-session change of 14.8% as the lowest, while the scale F suffered the highest betweensession change of 53.2%. The ANOVA result, for d.f. = 1 and 5, F - - 6 . 5 4 , and p < 0.05, indicated that the between-session change in ratings is significantly different among scales, but not between the two stimulus levels. Duncan's multiple range test revealed the homogeneous subsets of scales (p < 0.05) are Scales A, B, E and D; Scales B, E, D and C; Scales C and F. For discomfort level,

Scales A, E, D and B had less between-session change than Scales F and C. The ANOVA result (d.f. = 1 and 5, F = 8.69, p < 0.05) indicated that there are significant differences in between-session change among scales, but not between stimulus levels. Duncan's test gave the homogenous subsets as Scales A, E, D, B and F; Scales F and C. For overall discomfort, Scales D, B, A, and E suffered less between-session change as compared to Scales F and C, although the changes ( > 27%) are generally larger than those for pressure intensity ratings. Duncan's multiple range test showed that the homogenous subsets are Scales D, B, A, and E; Scales E and F; Scales F and C. In summary, Scale A showed the smallest between-session change for all three rated items. Scales B, E and D are almost at the same level, and Scales F and C are the least reliable in terms of between-session changes.

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W. Shen, K. C, Parsons /International Journal of lndustrial Ergonomics 20 (1997) 441-461 Table 6 Coefficients of variation (CV) of rating scores using the six scales (in %) Rated item

Scale

Intra-individual Session I

lntra-/inter ratio

Inter-individual Session 2

Session 1

Session 2

85

120

85

120

Avg

85

120

85

120

Avg

Pressure intensity

A B C D E F

14.6 18.9 39.3 33.7 34.9 25.5

10.0 11.7 16.4 23.4 18.3 21.2

10.9 16.1 20.9 36.3 21.7 21.9

8.6 10.4 17.3 15.2 20.9 27.7

11.0 14.3 23.5 27.1 24.0 24.1

21.4 28.6 53.2 48.4 40.5 50.3

23.3 22.0 36.2 26.2 28.2 57.1

27.0 29.2 40.1 47,9 40.6 57.8

21.7 23.9 24.6 40.2 25.4 47.3

23.3 25.9 38.5 40.7 33.7 53.1

47.3 55.1 61.0 66.7 71.2 45.3

Local discomforl level

A B C D E F

20.0 31.0 50.1 47.6 40.8 21.3

12.2 13.3 36.4 17.6 25.8 24.9

14.3 29.7 28.0 27.3 31.2 25.5

5.1 16.9 22.1 15.5 21.7 23.8

12.9 22.7 34.1 27.0 29.9 23.9

31.5 35.1 72.9 64.0 56.7 72.4

35.1 22.5 59.4 26.8 40.3 69.5

32.3 34.0 58.4 39.1 47.4 72.4

22.8 22.8 32.6 36.9 26.8 48.2

30.4 28.6 55.8 41.7 42.8 65.6

42.4 79.4 61.1 64,7 69.8 36,4

Overall discomfort

A B C D E F

24.0 22.4 51.0 27.7 43.5 40.3

16.4 13.0 20.0 17.3 39.3 28.0

18.0 35.9 29.4 38.3 43.0 43.0

11.6 18.5 21.3 18.2 24.5 26.2

17.5 22.4 30.4 25.4 37.6 34.4

45.0 40.0 9 l. 1 67.8 68.3 97.8

46.0 30.9 83.4 36.9 53.1 88.6

39.2 45.3 63.2 50.0 63.3 62.2

25.6 26.3 36.1 36.4 34.9 48.5

38.9 35.6 68.4 47.8 54.9 74.3

44.9 63.0 44.4 53.1 68.4 46.3

Note: lntraindividual CV was the mean of 12 subjects' individual CVs which were computed from the replicated ratings of each subject, and interindividual CV was computed from the averaged ratings of all subjects, lntra-/inter- ratio was the relative percentage of intraindividual CVs as compared to interindividual CVs.

Variability of ratings. Standard deviation of rating scores is commonly used to express the individual consistency in using a scale. However, since each scale has its own scaling dimension, the standard deviations cannot be compared directly. Coefficient of variation (CV), the ratio between standard deviation and the mean score, were thus computed. Table 6 summarises the coefficients of variation (in %) both intra- and inter-individually. Intra-individual CV depended on each subject's internal consistency in using a scale and is computed as the average of 12 subjects' individual CVs. Inter-individual CV was due to the heterogeneity of the subject group or the large between-subject difference in sensations, and was computed from the averaged ratings of all subjects. The intra-/interratio indicates the relative magnitude of intra-individual CVs as compared to inter-individual CVs. A small intra-/inter-ratio suggests high capability

of the scale in revealing individual differences in judgement. Scale A exhibited the smallest intra-individual and inter-individual variations among the six scales. Its intra-/inter- ratio is also small. Therefore this scale has the best internal consistency and is good for identifying individual differences in pressure sensation. Among the three rated items, the degree of variation is: pressure intensity < discomfort level < overall discomfort. Subjects had larger individual differences in judging discomfort than in judging pressure intensity. According to a twostage model of magnitude estimation (Gescheider, 1988), ratings of pressure intensity may depend more on the judgement at the sensory stage, while ratings of discomfort level depends more on the judgement at the cognitive stage which is more subjective. Scales C, D, E and F have more intraindividual variability, suggesting that subjects are

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W. Shen, K, C Parsons / International Journal o/" lndustrial Ergonomics 20 (1997) 441-461

less consistent in using these scales for rating pressure. Coefficients of variation in rating the higher stimulus level, 120 mmHg, were generally smaller than those for the lower pressure stimulus level, 85 mmHg. Since the mean ratings for the high level were larger than those for the low level, the decrease in CV suggests the standard deviations varied little across pressure stimulus levels. This property is relatively independent of the scale types.

4.2. Validity The validity of scales is examined on mean ratings of subject groups by looking at the functional relationships between the stimulus and sensations. Three aspects were investigated: • Intra-group stimulus range shift for the "absoluteness" of scaling, which is concerned with whether judgements of different intensity ranges presented to the entire subject group remain consistent over time. The smaller the shift, the more absolute and more valid the scale is in measuring stimulus; • Inter-group comparison, which tests the absoluteness of scaling over different subject groups presented with different stimulus ranges; and • Linearity of psychophysical functions. Intra-group stimulus range sh!ft. This tests the differences in mean ratings of the whole subject group between the high stimulus range and the low range. Ratings by group 1 in session 1 were merged with those by group 2 in session 2 to represent the high stimulus range, and ratings by group 1 in session 2 and by group 2 in session 1 were merged for the low range. Mean ratings on both stimulus ranges for each scale are depicted in Fig. 1. Clearly, the range effect exists in all six scales. Stimuli in the low range are judged higher than those in the high range. The degree of the range shift, however, varies among scales. Table 7 shows the mean rating shifts (in %) at each common stimulus level due to difference in stimulus ranges. Scale A is the best in terms of its small range shift with an average of about 9% for all three rated items, i.e. pressure intensity, discomfort level, and the overall discomfort. It therefore possesses an

excellent property of "'absoluteness" in pressure intensity and discomfort scaling. Scale C, D and E generally exhibited less "absolute" scaling properties. Scale F behaves differently at 85 mmHg from the other five scales. The mean ratings for the low range were actually smaller than the mean for the high range. This may suggest that Scale F has different properties due to its geometrically spaced anchors as compared to other scales. A two-factor repeated measures ANOVA was conducted, the two factors being pressure stimulus levels, 85 and 120 mmHg, and pressure stimulus ranges, high and low. Significance values are listed in Table 8. ANOVA results confirm the previous findings. All six scales were significantly sensitive to pressure stimulus levels. The range shift effect was not significant for Scales A, B and C. However, Scales D and E were significantly or strongly influenced by the range shift in all or some of the three rated items, which degrades their validity. Scale F was not significantly influenced by the range shift, but the interaction between stimulus levels and the range factor was significant for pressure intensity ratings. This interaction manifests a change of psychophysical functions with stimulus ranges. Inter-group comparison. Inter-group comparison assesses the difference in mean ratings between two subject groups, each experiencing a different stimulus rang e in the same session. Fig. 2 presents the mean ratings of the two subject groups in both sessions using the six scales. In both sessions, ratings of the two subject groups followed similar psychophysical functions, although the two groups were presented with different stimulus ranges. Table 9 summarizes the differences (in %) in mean ratings between the two subject groups, at two common stimulus levels in different ranges. In the first session, the inter-group range shifts were generally higher than the intra-group range shifts, as is seen by comparing session 1 in Table 9 with Table 7. For example, the differences in mean ratings between groups on Scale A were 9% at 85 mmHg and 15% at 120 mmHg, but the intragroup differences were only 6% and 7%, respectively. Comparing the six scales, Scale A exhibited the smallest inter-group difference in ratings. In the second session, ratings formed another pattern.

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Pressure intensity 85

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B C D E F

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452

W. Shen, K.C Parsons / International Journal oj'lndustrial Ergonomics 20 (1997) 441 46l

Table 8 Significance values for stimulus range shifts

Scale

A B C D E F

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Stimulus level

Range

S. Levelx range Stimulus Range level

S. Level x range Stimulus Range level

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Note: When the significance level is marked as 0.00, it indicates a value smaller than 0.01.

With the exception of the pressure intensity rating on Scale D, the differences in ratings between groups on the other five scales were remarkably reduced. Mean ratings on Scale A from both of the groups almost lie on the same line, which denotes that a unified psychophysical function governs the ratings even though the stimulus ranges are different. The differences in mean ratings for the intergroup comparison involve contributions from the range shift and the inter-group difference. Since the experimental conditions between the two sessions were the same except for a one week interval, the attenuation in differences may be due to the training effect in the first session. None of the 12 subjects had any experience in using the six scales for pressure related ratings prior to the first session. Correction of psychophysical functions due to m e m o r y is not likely to occur, because the two sessions were one week apart and the stimulus ranges were shifted. Bias due to range effect is therefore partially corrected by the rating experience in the first session. In summarizing the inter-group comparison, Scale A distinguishes itself from the others with the smallest between-group difference in ratings. Scales C, D and E suffered from large inter-group differences and range effects. A training effect is identifiable on all six scales. Relative differences in ratings between groups were greatly reduced in the second session.

Linearity and consistency ~[psychophysical Jimctions. G o o d linearity of the stimulus-sensation relationship implies that subjects are able to quantify and rate each stimulus on the scale, by following an inherent psychophysical function as predetermined by the structure of the scale. However, a scale with excellent linearity within one stimulus range may have a different slope of the relationship when the stimulus context changes. This problem of consistency can be examined through stimulus range shifts or through temporal shifts. Regression analysis can be an appropriate approach to define the linearity and consistency, but a graphic examination is sufficient for a comparison purpose. Figs. 1 and 2 show that Scale A had excellent log-linearity and consistency in its psychophysical function. Scale C exhibited fair linearity but poor consistency. The slope of the function was influenced by the stimulus range. Scale D was developed to possess a ratio property (Borg, 1982), but the result of this study did not support the assumption. The log-linearity was poor, although the consistency of its function is fair. Scale E suffered from severe centering bias, as the mid-level in the low stimulus range was rated higher than the mid-level in the high range. This bias was more prominent for ratings in the first session. Scale F was intended to be a ratio category scale and the result confirmed this property. After the mean ratings were log-transformed, linearity of the functions was excellent. When the consistency

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is examined, however, the scale behaves unsatisfactorily. The two lines from different stimulus ranges had different slopes, suggesting that subjects used inconsistent rules in making rating judgements. The validity of Scale F was thus reduced.

5. Discussion

5.1. Subjective preference of scales A questionnaire was administered at the end of the second session to obtain subjects' preference of

454

W. Shen, K. C. Parsons /International Journal of'Industrial Ergonomics 20 (1997) 441-461

Table 9 Relative differences in mean ratings between the two subject groups (in %) Rated items

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Session 2

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the scales. T h e six scales w e r e a r r a n g e d on a n o t i c e b o a r d a c c o r d i n g to t h e i r a l p h a b e t i c a l o r d e r . S u b j e c t s w e r e a s k e d to r a n k o r d e r e a c h o f the six scales in t e r m s o f scale clarity, p r e c i s i o n a n d o v e r a l l preference. T h e p u r p o s e of this q u e s t i o n n a i r e was to d e t e r m i n e w h e t h e r t h e scale c h o s e n o n the c r i t e r i a o f r e l i a b i l i t y a n d v a l i d i t y m a t c h e d the s u b j e c t i v e preference.

M e a n r a n k s c o r e s of s u b j e c t s ' scale p r e f e r e n c e s for e a c h c r i t e r i o n are d e p i c t e d in Fig. 3. F o r " c l a r i t y " , Scale B, A a n d E w e r e r a n k e d l o w e r a n d t h e r e f o r e b e t t e r t h a n the o t h e r scales. F o r " p r e c i s i o n " , Scale A was u n a n i m o u s l y r a n k e d the best. F o r ' o v e r a l l p r e f e r e n c e ' , Scale A was a g a i n d i s t i n g u i s h e d f r o m the others. Scale C was the least f a v o r e d in all t h r e e aspects. K e n d a l l ' s test ( F e r g u s o n a n d T a k a n e , 1989) o n t h e r a n k o r d e r s ( c h i - s q u a r e = 16.156, d.f. = 5, a n d p = 0.0064) s h o w e d a s i g n i f i c a n t difference a m o n g scales. Scale A h a d t h e l o w e s t r a n k , so it was the m o s t f a v o r e d scale. I n t e r e s t i n g l y , Scale E, o n e of the t w o visual a n a l o g u e scales, r a n k e d as the s e c o n d f a v o r i t e a l t h o u g h its reliability a n d v a l i d i t y w e r e f o u n d to be p o o r e r t h a n Scale B. 5.2. Effect o / s e s s i o n duration on sensation ratings" T h e session l e n g t h of a b o u t 90 m i n m a y h a v e c a u s e d f a t i g u e in the subject. C a r e was t a k e n in the e x p e r i m e n t a l d e s i g n to m i n i m i z e the c a r r y - o v e r

W. Shen, K.C. Parsons / International Journal of lndustrial Ergonomics 20 (1997) 441 46l

effect of each stimulus on the ratings of both the next stimulus and scale. The step test between two stimuli was also useful. Nevertheless, a catch-up rating at the end of each session was arranged to quantify any fatigue effect. Take the procedure for the first subject in the first session as an example. According to the design, this subject used Scale C to start the test session, followed by Scales B, E, F, D and A, with the stimulus ranging from 85, 120, and 165 mmHg. When the last scale, Scale A, was tested, Scale C was used again for the catch-up rating of the mid stimulus level, 120 mmHg. This rating was compared with the two initial ratings of the same stimulus level on the same scale. The design has ensured that, among each subject group, each of the six subjects used a different scale to start the session, and therefore each scale had a chance to be used at the end of the session. Since each scale had its own structure and scaling dimension, the equivalent standardized scores were calculated based on the sample means and standard deviations of the scale. The statistics for the standardized scores were extracted from the data used in reliability analysis. One-way analysis of variance did not show any significant difference in mean ratings between the beginning and the end of sessions, although the final ratings in a session tend to be higher than the initial ratings. Thus the session duration did not induce any significant fatigue effect.

5.3. Properties of individual scales The properties of the six scales are summarized in Table 10. Scale A. Scale A, the CP-50, behaved the best among the six scales for rating seated pressure sensation and the perceived discomfort. Initially developed for pain rating, the scale is actually a scaling procedure and has been tested in detail in terms of its reliability and absoluteness in scaling (G6bel et al., 1988; Ellermeier and Westphal, 1991). In a pilot experiment (Shen, 1993), the CP-50 scale was used to rate pressure sensation and discomfort level on the upper arm and under the thigh using a clinic sphygmomanometer. The intraclass correlation coefficient for a one week interval was 0.989 for intensity and 0.985 for discomfort. The

455

apparently low correlation in the current study was due to the confounding effect of stimulus range shift. Comparatively, the correlation of Scale A was the highest among all the scales. The equivalent intraclass correlation coefficient was 0.67. Considering the stimulus range difference, this ICC value shows very good reliability. The validity is best manifested by its smallest intra-group range shift and the inter-group difference in ratings. The small intra-group range effect indicates that subjects have used the scale in an absolute manner, and the ratings are made not in reference to the stimulus range, but to the subjects' own perceptual frame. Between two subject groups, the small difference enforced our hypothesis that subjects are referring to their experience for making judgements. The instruction of the category partitioning procedure was important in directing subjects' attention to their daily experience of pressure intensity and discomfort. The two-step rating sequence ensures appropriate categorization of the ratings while maintaining the discriminability. This was the intention when the original scale was developed. Its excellent reliability and validity as defined in our study imply that the scale can have wide application in intensity and discomfort ratings. As a numerical scale, the CP-50 would be very useful for both experimental studies of pressure discomfort and practical seating evaluation. Each category is clearly defined with equal intervals between categories and scale points. This allows for reliable judgements of both pressure intensity and discomfort. The subject in either an experimental test or a seating trial can easily look at the scale from a distance and choose a number that best describes his or her feeling at the moment, instead of having to mark, for example, on a visual analogue scale line. Scales B and E. Scales B and E, both of VASs, are largely of the same structure, except for the difference in the left anchors. To most subjects, Scale B has a rather odd end on the left: 'just noticeable" pressure or discomfort. "No sensation at all" was marked offthe scale line and recorded as 0, whereas a 'just noticeable" sensation was scored 0.5. Such a structure is problematic both in scaling method and for statistical treatment. A measure of 'just

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noticeable" sensation is a psychophysical and statistical concept. To provide a judgement of "just noticeable" sensation in a rating task is perhaps beyond human sensory discriminability. Statistically, opposed to the continuity of the remaining portion of the line, the gap between 0 and 0.5 disqualified parametric data processing. Both Scales B and E behaved poorly in the reliability test. The unusual structure however made Scale B appear to have higher validity than Scale E: the linearity of Scale B was maintained with different ranges of stimuli, and its consistency was kept throughout the whole range. In comparison, Scale E suffered a serious centering effect, the mid stimulus in low range being rated higher than the high range. In examining individual ratings, it was found that the poorer validity of Scale E was related to the scoring method. Ratings on Scale E was scored as the length from 0, "no at all", to the rated point, while on Scale B, the scores were measured from 0.5 "just noticeable" onwards. The increase in base score bv 0.5 unexpectedly corrected the linearity. Scale C. Scale C proved to be the worst rating scale for pressure intensity and discomfort. Behavior of this scale was very inconsistent. Subjects may have had difficulty finding a reference for their judgement. Sensitivity was also poor. Scale D. Explanation of Scale D, CR-10, was somewhat difficult. Borg (1982) developed this scale to replace the previous RPE-20 (Borg, 1962) for human sensations, intending to define more clearly the scale points in an absolute manner, and to introduce a ratio property. It has been increasingly applied to physical exertion or intensity rating (Johansson and Ljunggren, 1989; Thompson and Chaffin, 1993; Ulin et al., 1993), and to body discomfort rating (Gustafson-SiSderman, 1987; Grinten and Smitt, 1992; Bobjer et al., 1993; Kihlberg et al., 1993; WikstriSm, 1993). Results from this study however did not justify either of these two purposes. First, the reliability was not as good as it was assumed. Although the test-retest correlation was almost comparable to that of the CP-50 scale, the variation in ratings was very large, 42% according to our results. In reporting their study, Bobjer et al. (1993) provided the mean and standard deviations of the perceived discomfort of the hand, using the

457

CR-10 scale. The average coefficient of variation from their data was as high as 57.6%. Few significant results were elicited in their work due to the large variation in ratings. Secondly, ratings on the CR-10 exhibited large intra-group range shift as well as inter-group difference. Not only did each subject use his or her own "subscale" (Bobjer et al., 1993), but each individual was not using the scale in a consistent manner. The large inter-individual variation in the CR-10 rating may not reflect individual difference in sensation when such variation is due to the large intra-individual variability and to the poor validity of the scale itself (Thompson and Chaffin, 1993). Furthermore, a common trend in ratings using the CR-10 scale from the cited literature is that discomfort ratings are typically very low toward the minimal end, and the distribution is left skewed (Gustafson-SiSderman, 1987; Grinten and Smitt, 1992). Some of our subjects even complained about the ambiguity of scale anchor definitions, e.g. "somewhat". Scale F. Scale F was structured exactly according to the ratio qualities. However, the study showed that the ratings were skewed towards the low sensation end, similar to the trend found with Scale D. The test-retest reliability was rather poor due to large variation in the low-rating end. For validity, intra-group range shift was small but inter-subject group difference was large, indicating that subjects used this scale in a rather absolute way on an individual basis. When the ratings are log transformed, however, the linearity is excellent. The scale may especially be useful for high stimulus range or severe discomfort ratings. At the time of submitting this current study for publication, the first author read a paper by Kihlberg et al. (1995) where a slightly different 20-point discomfort scale was used. It would make an interesting postgraduate coursework to compare the psychometric properties of the two 20-point scales.

6. Conclusion

Scale A, the five-category 50-point scale, showed the best overall reliability and validity for pressure intensity and discomfort ratings. The scale is very sensitive in terms of discriminability, a quality to be

458

~ Shen, K.C. Parsons/International Journal of Industrial Ergonomics 20 (1997) 441 461

expected for psychophysical studies. Moreover, variation of ratings on this scale is basically constant, and the distribution of rating scores is normal, which justifies parametric statistical treatments. It is believed that the CP-50 scale would be appropriate for ratings on other intensity sensation and related subjective constructs.

52 51

very high preumr¢/severe discomfort

Acknowledgements The study was supported by a British Council award. The authors would also like to acknowledge the enormous assistance from the Institute for Consumer Ergonomics during this study. high pressure/severe discomfort

Appendix. Rating scales and instructions The original set of scales included a general instruction for the rating tasks. Each scale pattern was presented with two parallel scales, one for pressure intensity and the other for discomfort. For simplicity, the two scales are combined into one in this appendix.

Scale A You are now asked to rate the pressure stimuli by selectin9 numbers fiom the two scales below. Each scale is subdivided into 5 categories as we commonly use them in everyday life. Please start out by determining the category into which you think the pressure intensity or discomfort level falls. Then you may "fine tune" your judgement using the numbers within the category. Your rating is not limited by the largest number on the scales. Always consider the crude category as well as the number when you make your judgement. To give you an example. If you felt a pressure stimulus was high, and almost very high, the category "high pressure" comprises the numbers 31-40. Due to the tendency towards "very high pressure", fine tuning may result in choosing a number close to the upper category boundary, such as a 38 or 39.

50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31

30 29 28 27 26 medium preuur¢/discomfort 25 24 23 22 21

low pressure/.,light discomfort

very low pressure/slight dlscomfoll

no pressure/no discomfort

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

W. Shen, K+C. Parsons /'International Journal of Industrial Ergonomics 20 (1997)44l 461

459

Scale B You are now asked to rate the pressure stimuli by marking a backslash " / ' across the horizontal line of the scales below. Please read the words under the line first and then decide your rating. If you feel that there is no pressure or discomfort at all, write a zero "0" on the most left end of the scale.

just noticeable pressure /discomfort

medium pressure /discomfort

extreme pressure /discomfort

I

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I Scale C

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0 0.5 1 2 3

4 5 6 7

None at all Very, very low ptesatre/slight discomfort Very low presane/slight disconffort Low pressure/slight discomfort Moderate pressure/discomfort Somewhat high pressure/severe discomfort High pressure/severe discomfort

(just noticeable)

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8 9 10

Vet),, very high pressure/severe discomfort

•

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(almost max)

460

~ Shen, K.C. Parsons /International Journal of Industrial Ergonomics 20 (1997) 441 461

Scale E

You are now asked to rate the pressure stimuli by markin9 a backslash "/" across the horizontal line of the scales below. Please read the words on the scales first and then decide your rating. No pressure at all

Medium pressure

/discomfort

I Scale F

You are now asked to rate the pressure stimuli by choosin9 a number on the scales below. You are permitted to use decimals and also go beyond 20 or 0.5 if you feel necessary. Please read the words on the scale first and then decide your rating.

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0.5 . . . . . . . . . . . . . . . . . . .

0

V©ry high premur©/s©verc discomfort

High preuur©/severe all.comfort

Medium preuute/discomfort Low preHuse/slight discomfort Very low preuure/slight discomfort . . . . . . . . . . . . . . . . . . .

No pressure/disoomfo~l

References Bobjer, O., Johansson, S.-E., Piguet, S., 1993. Friction between hand and handle. Effects of oil and lard on textured and non-textured surfaces; perception of discomfort. Applied Ergonomics 24, 190-202.

/discomfort

1

Extreme pressure /discomfort

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