A procedure to determine the individually comfortable

ERGONOMICS, 1999,

VOL.

42,

NO.

4, 535 ± 549

A procedure to determine the individually comfortable position of visual displays relative to the eyes WOLFGANG JASCHINSKI*, HERBERT HEUER and HANNEGRET KYLIAN Institut fuÈr Arbeitsphysiologie, Ardeystr. 67, D-44139 Dortmund, Germany Keywords: Viewing distance; Vertical gaze inclination; Visual strain. In an intervention phase, 38 operators used four diŒerent imposed screen positions (near versus distant, high versus low) for a full working day to experience the advantages and disadvantages. Screens at about 66 cm induced more reported strain than screens at about 98 cm. When operators later freely selected their individually most comfortable screen position, individually diŒerent changes due to the intervention were observed: some subjects changed to shorter, others to longer viewing distances, some operators adjusted the screen lower, others higher. These eŒects were con® rmed in repeated tests. Thus, trying out diŒerent screen positions appears useful for arranging the VDU workstation to the individually most comfortable screen location relative to the eyes.

1. Introduction At workstations with visual display units (VDU) a short viewing distance and a high screen relative to the eyes can induce visual strain (Jaschinski-Kruza 1988, 1991, Bergqvist and Knave 1994, Lie and Fostervold 1995, Lie et al. 1997, Jaschinski et al. 1998a). However, a general recommendation of a particular screen position for all subjects seems inappropriate since we observed marked individual diŒerences in viewing distance and screen height that subjects found most comfortable, and therefore preferred during work, when they used the ¯ exibility of freely adjustable workstations (Jaschinski et al. 1998a). We therefore suggested that each VDU user should place the visual display to a position that he or she prefers. This recommendation is physiologically plausible since subjects with normal vision diŒer reliably in several eye muscle functions that may be related to visual strain. For example, subjects with a distant adjustment of ocular vergence at rest (dark vergence) reported more visual strain and had a reduced performance in nearvision tasks (Owens and Leibowitz 1983, Tyrrell and Leibowitz 1990, JaschinskiKruza 1991, Best et al. 1996, Jaschinski 1998); further, they preferred longer viewing distances (Heuer et al. 1989). In some subjects, the resting vergence angle increases, i.e. the corresponding vergence distance shortens as the vertical gaze direction is declined (Heuer and Owens 1989, Heuer 1993). Those subjects who showed larger vergence errors at near vision spontaneously adopted longer viewing distances in the course of a near-vision task (Jaschinski 1997, 1998). Vergence errors also depend on the angle of vertical gaze inclination in an individually characteristic way (Jaschinski et al. 1998b).

* Author for correspondence. 0014± 0139/99 $12.00 Ó 1999 Taylor & Francis Ltd

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W. Jaschinski et al.

These previous investigations led us to the assumption that (individual) physiological conditions that may induce strain are avoided by operators after they had experienced more comfortable workplace conditions. Thus, the subjectively preferred adjustment may represent an ergonomically favourable workplace, provided it is physiologically plausible. In the present study, we investigate whether the following procedure can help the operators to ® nd their most comfortable screen position under realistic workplace conditions. During an intervention the subjects are required to use four widely diŒerent screen positions; these were a distant and low screen (A), a distant screen at eye level (B), a near and low screen (C) and a near screen at eye level (D). After having experienced the advantages and disadvantages of these screen positions, subjects were encouraged to adjust the screen to a position that they found most comfortable. 2. Method 2.1. Task of the operators and the workplace conditions This ® eld study was made in a telephone directory-assistance o ce of the Deutsche Telekom AG in Dortmund, where the operators provided customers with telephone numbers on request by phone. During this task the operators viewed the VDU almost continuously; no paper documents were used (for details, see Jaschinski et al. 1998a). We used three o ce rooms with a total of 72 technically identical, freely adjustable workstations (® gure 1). The operators changed everyday among these workstations as determined by the usual o ce organization; thus, at the beginning of work they found the workstations as adjusted by their predecessor. They kept the same workstation during the working day. The keyboard and the VDU were on separate tables that could be moved up and down under motor control. The viewing distance was adjustable by shifting the VDU table on a slide and by moving the

Figure 1. Workstation with adjustable table heights for both keyboard and screen so that subjects could freely adjust the viewing distance, d, and the angle, c , of vertical gaze inclination.

Eye and VDU position

537

VDU on the table. The screens could be moved horizontally by 50 cm and vertically by 32 cm. The VDUs had dark characters on a bright background with adjustable luminance. The characters were 4.5 mm high, which agrees with the recommended character height in terms of visual angle (20 ± 22 min arc) at about 74 cm viewing distance (ISO 1992). 2.2. Measurement of screen position relative to the eyes As in Jaschinski et al. (1998a), we measured several parameters with an anthropometric ruler and a protractor with a pendulum, including eye level above the ¯ oor, viewing distance from the eyes to the screen (i.e. to a reference point near the area of the screen at which the operators looked most of the time), inclination of the VDU surface and height of the VDU housing above ¯ oor. From these measures we calculated the angle, c , of vertical gaze inclination between horizontal and the line connecting the eye and the reference point on the screen surface (de® ned as negative when the eyes look downwards, see ® gure 1). We collected ® ve measurements per day at intervals of about 1 ± 2 h. 2.3. Questionnaires The subjective feeling of fatigue was investigated after work. A ® rst questionnaire, translated to English in table 1, was administered each day and comprised three subsets of items on eyestrain, headache symptoms and musculo-skeletal strain (each with a 7-point scale). For each subset, the numbers indicated by the subjects were added. Additionally, during phases I and III general fatigue was evaluated with 15 items of the factors `readiness to work’ , `fatigue’ and `sleepiness’ (Nitsch and Udris 1976). The responses on a 6-point scale were ® rst transformed to have positive numbers for conditions of fatigue for each item and then added to give an indicator of general fatigue. Table 1. Items and scales of the questionnaire on eyestrain, headache symptoms and musculo-skeletal strain. Eyestrain 1. I have di culties in seeing. 2. My eye lids are heavy. 3. I feel eye strain. 4. I have burning eyes. 5. I have a strange feeling around the eyes. 6. I have itching eyes. Headache symptoms 1. I feel numb. 2. I feel dizzy. 3. I have a headache. Musculo-skeletal strain 1. I have pain in my arms. 2. I have pain in my neck. 3. I have pain in my back. 4. I have pain in my shoulders. These items had the following scale: not at all = 1 2 3 4 5 6 7 = yes, very much.

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2.4. Time scheme and procedure of the study The daily working time diŒered within and among subjects between 6 and 8 h, depending on the current o ce organization. The study comprised three phases (I, II and III) within a 9-week period. Phase I was made to investigate the status quo; thus, the subjects were instructed to work as usual. No information was given that the preferred screen position may be individually diŒerent. Each subject was tested on up to 4 experimental days (denoted as days I 1, I 2, I 3 and I 4) at about 1-week intervals. Between these experimental days, the operators had ordinary working days. Phase II was the intervention. The subjects were instructed to maintain four imposed positions of the VDU relative to the eyes for a whole working day (® gure 2). The order of these four conditions was permutated across subjects. The experimenters attached a paper measuring tape at each VDU that allowed the operators to measure the viewing distance and the height of the VDU table above ¯ oor level. The subjects were instructed to do these measurements three times per day. This was made to con® rm the adjustments made by the subjects. At the beginning of phase II, the operators were informed about the procedure of the intervention with a printed hand-out including the illustrations of ® gure 2, the questionnaire of table 1 to be completed after work, a form for the table height and viewing distance, and informations on the procedure summarized shortly as follows: This instruction may help you to place the screen in a way to reduce strain at work. We ask you to try out four diŒerent screen positions on the following four working days to let you experience which are favourable for you or not. The preference of working conditions may diŒer among individuals; thus, do not care about what others say, it depends on your own opinion. You may ® nd some imposed screen positions unusual in the ® rst moment, but nevertheless try them for the whole working day since one sometimes quickly rejects an unusual condition that may turn out to be favourable in the long run. For a near screen, move the screen most closely to the eyes to a viewing distance of about 50 cm; for a distant screen, move the screen as far as possible to a viewing distance of 90 ± 100 cm. For a high screen, adjust the screen to look horizontally, but not upwards; for a low screen, adjust the screen as low as possible.

Figure 2. Illustration of the four experimental conditions used in the intervention (phase II): low and distant screen (A), distant screen at eye level (B), near and low screen (C), and near screen at eye level (D). The actual dimensions are given in table 2c.


539

With this written information, the operators managed all procedures of the intervention themselves without assistance by the experimenters. We did this because we were interested in possible eŒects of the intervention without personal advice of the experimenters. Phase III was made to see whether subjects changed their screen position due to their experience during the intervention and whether the individual screen positions were stable over time. Subjects were instructed at the beginning of phase III to adjust the screen to a position that they preferred. The experimental procedure was the same as in phase I: subjects were tested once a week over a 4week period. 2.5. Statistical analysis The BMDP Statistical Software (Dixon 1992) was used for most tests. The questionnaire data of phase II were analysed with a two-factor analysis of variance with repeated measures on ranks (Huber 1982). 2.6. Participants From the 45 participants, seven were not included into data analysis for two reasons. We excluded subjects who did not reach a minimum visual acuity of 1.0 (in decimal units) in the left and right eye in the range of viewing distances used at work, as measured with the Binoptometer (Oculus, Wetzlar, Germany). Presbyopia, i.e. the inability to focus near objects with increasing age and the corresponding near-vision glasses, may restrict possible screen positions; thus subjects > 45 years were not included. The remaining sample of 38 operators had a mean 6 SD age of 30 6 8 years (range 21 ± 44).

3. Results 3.1. Phase I (pre-intervention phase) We calculated the mean screen positions for each subject from the ® ve measurements per day. Table 2 summarizes the group means, SD and ranges for the 4 experimental days (about one per week) in phase I respectively. Table 3 shows highly signi® cant test ± retest correlations between the 4 experimental days of phase I over nearly the full range of viewing distances and vertical gaze inclinations that were possible at these workstations (table 2). For the gaze inclination, we also calculated the partial test ± retest correlations in which the eye level above ¯ oor was included as a covariable to remove its eŒect. These diŒered in no case by > 0.02 from the original correlations in table 3a. Thus, the reliability of the vertical gaze inclination is not an eŒect of diŒerent eye levels in short or tall subjects at an unchanged screen height above ¯ oor. 3.2. Phase II (intervention phase) The mean viewing distance changed from 66 to 98 cm between the near and distant condition and the VDU table was elevated by about 17 cm from the low to the high screen condition (table 2). A rank analysis of variance with the factors viewing distance and gaze inclination showed that near screens induced signi® cantly greater visual strain (p < 0.01), musculo-skeletal strain (p = 0.02), and headache symptoms (p < 0.01) than distant screens; the factor gaze inclination and the interaction were not signi® cant (® gure 3).

540


3.3. Phase III (post-intervention phase) We found no signi® cant diŒerences between phases I and III in viewing distance (t = 0.04, n = 38) and vertical gaze inclination (t = 0.95, n = 38); this can be seen from the very similar means before and after the intervention in table 2. Thus, Table 2.

Description of screen position on the experimental days of phases I, II and III. Viewing distance (cm) Mean 6

Phase 1 Day I 1 Day I 2 Day I 3 Day I 4 Phase III Day III 1 Day III 2 Day III 3 Day III 4

SD

Range

Gaze inclination (deg) Mean 6

SD

10.46 11.06 11.76 11.86

3.6 3.6 3.9 3.9 Ð

11.36 10.76 10.46 10.56

3.4 3.6 3.8 3.6 Ð

89.5 6 89.3 6 89.3 6 89.4 6

10.4 9.6 11.2 10.8

68 68 67 68

... ... ... ...

109 107 110 107 Ð

90.1 6 89.8 6 88.9 6 89.1 6

8.9 8.3 8.7 8.5

66 67 66 61

... ... ... ...

105 106 100 102 Ð

Ð

Ð

Ð

Ð

Ð

Ð

Range

Ð Ð Ð

Ð Ð Ð

22.9 18.7 22.5 21.9

... ... ... ... Ð

20.9 20.1 20.6 18.9

... ... ... ... Ð

Ð Ð Ð

Ð Ð Ð

Sample size 4.7 4.1 6.2 4.9

38 38 35 27

4.1 5.5 1.2 3.9

38 38 36 33

Height of VDU-table (cm) Phase II (Intervention) Condition A 98.9 6 Condition B 97.5 6 Condition C 67.2 6 Condition D 65.4 6

Table 3.

5.5 5.5 11.0 9.5

91 83 52 48

... ... ... ...

115 116 89 86

61.36 77.16 60.36 78.16

8.5 4.9 3.6 5.7

51 66 53 67

... ... ... ...

100 89 67 99

38 38 38 38

Correlations between the experimental days of phases I and III for viewing distance and vertical gaze inclination. All correlations are highly signi® cant.

Phase I

Day I 1

Day I 2

Day I 3

Viewing distance Day I 2 Day I 3 Day I 4

0.84 0.86 0.84

0.96 0.95

0.92

Gaze inclination Day I 2 Day I 3 Day I 4

0.86 0.85 0.88

0.85 0.85

0.86

Day III 1

Day III 2

Day III 3

Viewing distance Day III 2 Day III 3 Day III 4

0.87 0.75 0.79

0.76 0.82

0.87

Gaze inclination Day III 2 Day III 3 Day III 4

0.82 0.77 0.88

0.77 0.86

0.81

Phase III


541

averaged across subjects, the intervention induced no uniform change in the preferred screen position. However, 16 of 38 subjects indicated in a questionnaire (completed after intervention) that they had learned something new about the

Figure 3. Questionnaire results before and after the intervention and in the four conditions of the intervention (median6 half the interquartile ranges (Q3 Ð Q1 )/2) with levels of signi® cance.

542


comfortable placement of the VDUs and 13 operators indicated that they would change their screen position, however, in diŒerent directions in diŒerent subjects. These individual eŒects were tested as follows. The complete sample of 38 subjects participated in the ® rst and in the second week of phases I and III respectively. Thus, we can compare the eŒects in the ® rst weeks (day III 1 Ð day I 1) with those in the second weeks (day III 2 Ð day I 2). These two diŒerences are independent since they result from experimental days, separated by about 1 week, while the operators had changed the workstations each day in the meantime. Further, these diŒerences were chosen to have no common computational component to exclude the possibility of arti® cial correlations. The positive and negative diŒerences in the scatter plots of ® gure 4 show that after the intervention the viewing distance was increased by some subjects and decreased by others, and that some raised the gaze inclination while others lowered it. Data points close to the origin of the graphs indicate no change. Signi® cant correlations between these changes were found for viewing distance (rs = 0.53, p < 0.0005) and for gaze inclination (rs = 0.42, p < 0.005). These are Spearman rank correlations rs in order not to overestimate the correlation because of the outlier in ® gure 4b. Thus, the intervention induced changes in the screen position in diŒerent directions in diŒerent subjects; these changes were reliable since they were observed repeatedly. Further, we tested the stability of the eŒect over the complete post-intervention period of 4 weeks (® gure 5); this analysis is only possible for 24 complete cases because of missing values. The subjects were equally distributed in three subgroups depending on the change in viewing distance that they showed from immediately before to immediately after the intervention (day III 1 Ð day I 4; open symbols). Subgroup a comprised the eight subjects with a decrease in viewing distance of

Figure 4. Changes in viewing distance (a) and vertical gaze inclination (b) from before to after the intervention, are correlated between the ® rst and the second week of phases I and III, i.e. (day III 1 Ð day I 1) versus (day III 2 Ð day I 2). Spearman rank correlation rs is indicated.


543

> 1.1 cm and subgroup c the eight subjects with an increase of > 3.0 cm; the remaining eight subjects in subgroup b had intermediate changes close to zero. If the intervention had a persistent eŒect, this grouping on the basis of the ® rst day after the intervention should also apply to the later weeks of phase III. As expected, subgroup a had shorter viewing distances also in the later weeks of phase III compared with phase I (shown by the downward arrow). The inverse eŒect can be observed in subgroup c which changed to longer viewing distances after the intervention (shown by the upward arrow). Subgroup b also showed no change on the following days. The analysis of variance with repeated measures including the factors `intervention’ (pre-, post-) and `time’ (3 days; closed symbols) and a grouping factor (subgroups a, b and c) showed a signi® cant interaction between intervention and grouping (F(2,21) = 3.74, p = 0.0409). Thus, the direction of change in viewing distance that was made in the ® rst week persisted during the following 3 weeks after the intervention. The amount of change in viewing distance appears to be much larger on the ® rst post-intervention day than later (open symbols). However, this is explained by the regression eŒect: the groups have been formed on the bases of a large diŒerence from the day before to the day after the intervention; thus any change in viewing distance observed in this diŒerence that is random and not produced by the intervention will not be included in the following days. To test a possible time eŒect from the ® rst to the second day after the intervention, we calculated the absolute values of the changes from days I 1 to III 1 (5.7 6 4.5 cm, mean 6 SD) and from days I 2 to III 2 (4.9 6 3.8 cm). These changes were not signi® cantly diŒerent (t = 0.78, paired t-test, n = 38). Further, time or the interaction between time and intervention were not signi® cant over the corresponding 3 days in the 24 complete cases. Therefore, the eŒect of the intervention on the viewing distance appears stable during the 4 weeks after the intervention. The same analysis as described in ® gure 5 was also made for the vertical gaze inclination. The results showed essentially the same pattern as observed for the viewing distance; however, the interaction between the grouping factor and the intervention factor did not reach signi® cance (F(2,21) = 2.46, p = 0.1095). During phase III, when the operators have presumably found their individually most comfortable screen position, we found highly signi® cant test ± retest correlations between the 4 experimental days (table 3) showing that both viewing distance and vertical gaze inclination were reliable individual measures. To describe the intra- and interindividual variability, we calculated the SD of the up to four daily means in phase III for each subject; these were 3.3 cm for viewing distance and 1.58 for gaze inclination, averaged in the group. These mean intraindividual SDs were less than half the interindividual SDs in the group of 38 operators, which were 7.8 cm and 3.38 respectively. Thus, individual operators chose screen positions in a range much smaller than the interindividual range, as shown by the examples in ® gure 6. The screen positions before and after the intervention (averaged across the 4 days in phases I and III respectively) were highly correlated for viewing distance (r = 0.84, n = 38) and for vertical gaze inclination (r = 0.74, n = 38). This suggests that the individual diŒerences in preferred screen positions persisted over 9 weeks. One may expect that the visual acuity determines the viewing distance at a constant character height on the screen. However, visual acuity, expressed as the maximal acuity of the two eyes, was not correlated with the mean preferred viewing

544


Figure 5. Viewing distances in the course of phases I and III, separately for three subgroups (a), (b) and (c) of eight operators each that were selected to have a large positive, intermediate and large negative change in viewing distance from immediately before to immediately after the intervention (day III 1 Ð day I 4; open symbols). Horizonzal lines illustrate the mean viewing distance over the corresponding 3 days in each group.


545

Figure 6. Preferred screen positions relative to the eyes during phase III. Each symbol respresents the individual daily mean value of the 4 experimental days: open symbols for all subjects and closed symbols for four individuals.

distance during phase III (r = Ð 0.07). The acuity ranged between 1.0 and 1.6, which is considered to be su cient for work at VDU workplaces. The questionnaire results did not change signi® cantly during the series of 4 experimental days of phases I and III, respectively, as shown by Friedman two-way analyses of variance. To test the eŒect of the intervention we calculated for each subject the median scores of all days of phases I and III, respectively. After the intervention the scores were signi® cantly lower for eyestrain (p = 0.003, one-tailed Wilcoxon tests, n = 38), but not for musculo-skeletal strain (p = 0.092) and headache symptoms (p = 0.139) (® gure 3). General fatigue was signi® cantly lower after the intervention than before (p = 0.0278). One should also expect that the preferred screen positions in phase III induced less symptoms than the four imposed settings during the intervention phase. This was con® rmed for the two conditions C and D with near screens that induced signi® cantly more eyestrain, musculo-skeletal strain and headache symptoms than the preferred screen positions in phase III (see ® gure 3 for the levels of signi® cance following the Wilcoxon test). The long viewing distances in conditions A and B did not lead to more strain than the preferred settings in phase III. 4. Discussion We had operators use short versus long viewing distances and high versus low screens to see whether having experienced the resulting advantages and disadvantages may help the operators to adjust the individually most comfortable screen position. Due to this intervention some subjects shortened and others increased their viewing distance; some changed to higher, and others to lower screens. These individual changes in diŒerent directions showed signi® cant test ± retest correlations in repeated measurements separated by about 1 week. The individual changes of the preferred viewing distance persisted over the 4 weeks after the intervention. These

546


results con® rm our expectation that the experience of comfort or discomfort when working with diŒerent, widely diŒerent screen positions can help to ® nd out the individually preferred screen position. The stability of the changes in screen position over the 4 weeks suggests that the operators received a bene® t from the intervention; this eŒect appeared to be more clear for viewing distance than for screen height in the present conditions. Each tryout phase during the intervention lasted 1 working day for practical reasons. Thus, the four conditions were administered within 1 week, which may not be too long for subjective comparisons of the conditions. It cannot be excluded that some subjects may require longer periods to accustom to a new screen position; but, at least, some operators complained about near screens already after a few hours or only minutes. Despite the reliable changes due to the intervention we found that the preferred screen positions were highly correlated already during phase I and between phases I and III. Apparently, most operators were aware of their preferred screen position already before the intervention. Accordingly, 35 of 38 operators indicated before phase I to adjust the screen everyday to an individually comfortable position. These observations agree with previous results (Jaschinski et al. 1998a) and were apparantly an eŒect of the organization in these o ces: since operators changed everyday between diŒerent, but technically identical and freely adjustable workstations, they found the workstation adjusted to a diŒerent position by the actual predecessor and, thus, they were able to experience more or less comfortable conditions. This schedule can implicitly help to ® nd out the individually preferred position, in a similar way as our systematic guidance in the intervention had caused operators to change their preferred screen setting. In hindsight, it is clear that since most operators knew their individually preferred screen position already, the eŒect of the intervention could not be expected to be large. For operators without this prior knowledge we expect larger eŒects of an intervention. Nevertheless, 16 of 38 operators indicated in a questionnaire (completed directly after the intervention) that they had learned something new about the workplace adjustment. Depending on VDU position, re¯ ections of lamps or windows may appear on the screen surface which can be avoided by adjusting screen height and tilt angle. This factor cannot explain the present results since the operators changed everyday between workstations with diŒerent locations in the o ce. Thus, a screen position that may be useful to avoid re¯ ections at one workstation might not help at another one. The observed means of preferred viewing distance (90 cm) and vertical gaze inclination ( Ð 108 ) and the large ranges agree well with results in Jaschinski et al. (1998a) and previous investigations summarized by Grandjean (1987) at realistic VDU workstations and in an experimental setting (Heuer et al. 1991). However, Heuer (1993), Lie and Fostervold (1995) and Lie et al. (1997) reported evidence that much stronger gaze inclinations of Ð 30 to Ð 458 may be advantageous (cf. Straker et al. 1997). This diŒerence may partly be explained by the fact that in the present conditions rather long viewing distances were used where the preferred screen position is higher than at near (Hill and Kroemer 1986, Kroemer and Hill 1986). Further, most operators did not try out the lowest possible adjustment of the VDU table during the intervention (which was 50 cm; table 2c); thus, they could not experience possible advantages of the lower gaze angles.


547

Viewing distances in the range 60 ± 110 cm may appear rather long; however, they are physiologically plausible since the accommodation and convergence systems of the eyes have their average resting positions near 1 m, with an interindividual range of about 50 cm to in® nity (Owens and Leibowitz 1983). Any shortening of the viewing distance relative to these individual resting positions increases the load on the ocular muscles and may contribute to eyestrain (Jaschinski-Kruza 1988, 1991, Heuer et al. 1989, Jaschinski 1998). Also in the present study, higher complaints were found at mean viewing distances of 66 cm than at 98 cm. The preferred vertical gaze inclinations observed were in the range of Ð 5 to Ð 208 downwards. These values overlap with the interindividual distributions of the resting inclination of the eyes (Levy 1969, Vaegan 1976, Leibowitz et al. 1983) and gaze inclination of minimum perceived exertion (Menozzi et al. 1994). Thus, some visual functions diŒer among subjects with normal vision in a way that suggests a physiological basis for the individual diŒerences in preferred screen position. Future research may try to predict the individually preferred screen position from individual visual functions. If subjects change the screen to a more comfortable position due to the intervention, one should expect a concurrent reduction in subjective symptoms of strain. After the intervention, less eyestrain and general fatigue was reported than before. However, this eŒect was not signi® cantly diŒerent in two subgroups that were built a posteriori to have stronger and minor changes in screen position due to the intervention. Thus, it is likely that the subjects’ expectation of bene® t from this ergonomic study may have in¯ uenced the reports in the questionnaire. But, a placebo-eŒect cannot fully account for the questionnaire results since, ® rst, not all questionnaires indicated an alleviation of complaints (musculo-skeletal strain and headache symptoms were not signi® cantly aŒected) and, second, a placebo-eŒect should be large immediately after the intervention and decay later, but we found no time eŒects over a 4-week period. It is clear that the bene® t of rearranging workstations according to the individual preferences will strongly depend on the preexisting conditions. At the present o ce, the operators had very ¯ exible workstations and many used these facilities already before the intervention to adopt their preferred settings. The reduction of strain will be the larger the more the workplace conditions are restricted. Especially in o ces where no viewing distances > 60 cm are possibe because of limited space on conventional tables, many subjects will pro® t from shifting the screen backwards since near screens can induce strain. In conclusion, most subjects prefer to place the screen relative to the eyes in a rather small intraindividual range within a much larger interindividual range (® gure 6). Given that individuals diŒer reliably in the screen positions that they ® nd comfortable, recommendations of a particular average viewing distance and vertical inclination of gaze for all subjects Ð as in many guidelines Ð may provide an ergonomic solution for many, but not for all users. As a systematical method to ® nd the individually most comfortable screen position we suggest a procedure like the intervention described above. The present task involved only one visual object, i.e. the VDU. If paper documents or other visual targets should occur, gaze shift between these diŒerent visual objects at diŒerent locations may require special ergonomic solutions (e.g. Jaschinski-Kruza 1990). Further, the inability to focus near objects with increasing age and the corresponding use of near-vision glasses may restrict possible screen positions; these conditions of presbyopia were not addressed in the present sample since no subjects were > 45 years.

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Acknowledgements We thank the Deutsche Telekom AG Dortmund for the cooperation, the operators for participating in the study, Professor C. R. Cavonius for comments on the manuscript, Dr M. SchuÈtte for the rank ANOVA, and Marion Luhmann, Eva Strzelec and Rainer FloÈring for doing the measurements.

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