VDU-screen filters: remedy or the ubiquitous Hawthorne effect?

International Journal of Industrial Ergonomics 27 (2001) 107–118

VDU-screen filters: remedy or the ubiquitous Hawthorne effect? Knut Inge Fostervolda,*, Erik Buckmannb, Ivar Liea,y b

a University of Oslo, Department of Psychology Box 1094, Blindern N-0317 Oslo, Norway Medical Division for the National Insurance Administration and Local Social Insurance Offices in Oslo, Drammensveien 60 N-0241 Oslo, Norway

Received 7 June 1999; received in revised form 23 September 2000; accepted 4 October 2000

Abstract The claim that visual display units (VDU)-screen filters improve health by reducing exposure to electrical fields and/ or improving screen legibility was investigated by introducing VDU screen filters into an office environment. The study design was quasi-experimental, with the first part conducted according to a simulated control group design with one intervention and one control group. In the second part, which served as a partly switched replication, the control group also received screen filters. The first group continued their use. The dependent measures were sick leave, productivity. subjective symptoms, and musculoskeletal condition. Subjectively reported symptom improvement was found in the group that received screen filters during the initial part of the study, but that was followed by an aggravation of symptoms during the second part for the same group. No effect was observed in the control group in either part of the study. The discontinuity observed between the study parts indicates that factors other than VDU-screen filter use were responsible for the results. However, the results appear to be explained by subject reactivity, i.e. the Hawthorne effect. The results caution against premature conclusions drawn from short-term interventions and emphasise the need for a better understanding of the various psychological and task-related factors that influence subjects.

Relevance to industry Visual display units (VDU) are widely used in industry. Knowledge of potential VDU-related health hazards can improve workers health and performance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Video display units; VDU-screen filters; Electrical fields; Hawthorne effect; Reactivity

1. Introduction

*Corresponding author. Tel.: +47-22-85-52-33; fax: +47-2285-44-19. E-mail address: [email protected] (Knut Inge Fostervold). y Deceased, January 1999.

The extensive use of information technology (IT) and video display units (VDUs) has led to a growing concern about potential health risks from prolonged VDU work. One risk factor that has received increasing attention is exposure to the electrical fields that surround VDUs and other electric devices. Among the more frequently

0169-8141/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 0 0 ) 0 0 0 4 2 - 1

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discussed health-problems associated with this exposure are ocular discomfort, skin symptoms, cataractogenesis and teratogenesis (Bergqvist, 1984; Smith, 1987). At present, there is no evidence of increased risk of adverse pregnancy outcome associated with VDUs (Bergqvist, 1984; Lide´n and Berg, 1991; Bramwell and Davidson, 1993; Breysse, 1995). This is also the case with cataractogenesis (Bergqvist, 1984). Scientific reports discussing skin problems (Swanbeck and Bleeker, 1989; Lide´n and Berg, 1991; Oftedal et al., 1995; Stenberg, 1995; Stenberg et al., 1995) and ocular discomfort (Bergqvist et al., 1992; Gur et al., 1994; Piccoli et al., 1996; Rechichi and Scullica, 1996; Jackson et al., 1997) are inconclusive at present. Over the recent years, the focus on possible negative health effects from exposure to electrical fields has shifted from specific symptoms to symptom clusters associated with the term ‘‘hypersensitivity to electricity’’. People suffering from such hypersensitivity are experiencing symptoms in the presence of an increasing number and variety of electric components and other electric sources. However, no specific symptoms or symptom clusters seem to precede these hypersensitivity outbreaks, and the composite of complaints is heterogeneous, with large individual differences (Lide´n and Berg, 1991; Bergqvist, 1995; Blomkvist and Almgren, 1995). Nevertheless, the cause is often attributed to one specific source, with the vast majority identifying the VDU. At present, little is known about the mechanisms that initiate this syndrome, but it most likely involves some sort of sensitisation that results in increased vulnerability to external and/or internal stimuli. Given the present knowledge, a causal link between exposure to electric fields emanating from VDUs and different health problems can not be refuted. Public awareness and concern have led manufacturers to develop devices designed to reduce electrical fields. Technical tests and measurements show that high-quality VDU-screen filters reduce electrostatic fields. In some cases, dependent on the VDU’s shielding properties, they also reduce AC-fields. Only minor reductions have been measured for electromagnetic fields (Hansson and Eriksson, 1988; Oftedal et al., 1995; Olsen and Feragen, 1996; Hedge et al., 1996).

In addition to reduced exposure to electrical fields, manufacturers claim that their filters reduce visual fatigue and asthenopic symptoms by reducing glare and improving screen legibility. Screen legibility and glare are important factors influencing ocular and visual symptoms associated with prolonged VDU-work (Collins et al., 1990; Bergqvist and Wibom, 1993; Hedge et al., 1995; Hedge et al., 1996). Empirical studies investigating the relationship between VDU-filters and health are scarce, but remedial effects have been reported in some papers (Oftedal et al., 1995; Hedge et al., 1996; Hladky and Prochazka, 1998). Unfortunately, the treatment intervals used in these studies were relatively short – between two weeks and one month. Epidemiological studies and studies using longer time intervals do not confirm those findings. In a large sample study including more than 25,000 subjects, Scullica and Rechichi (1995) found no differences in asthenopic complaints among VDUworkers using VDU-screen filters compared with workers without filters. Bergqvist and Wibom (1993) reported increased aches and discomfort associated with VDU-filters in a cohort study conducted over a two-year period. Thus, it is possible that the short treatment intervals are the key factor in studies reporting remedial effects. In order to explain the relationship assumed between treatment interval and remedial effect, it might be worthwhile to consider sources of reactivity such as the Hawthorne effect, a well-known confounder in quasi-experimentation. The Hawthorne effect is a psychological phenomenon that refers to relatively short-lived changes in subject’s behaviour caused by their awareness of being subjects in experiments. Despite increasing scepticism regarding the empirical foundation of the effect (Cook and Campbell, 1979, p. 66; Adair, 1984; Diaper, 1990; Jones, 1992), it is evident that subject reactivity is present in some experiments. In a review-article, Adair (1984) found no evidence of reactivity due to the following variables: (1) novelty alone, (2) special attention from significant others, (3) the mere knowledge of being subject in an experiment and (4) encouragement of success, all variables which have been held responsible for the Hawthorne

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effect. Moreover, he found evidence of reactivity in experiments where (1) the dependent variable did not involve well-practised skills, (2) subjects identified a purpose for the independent variable and (3) where expectations of personal behaviour was the identified outcome of an experimental situation. Moreover, Cook and Campbell (1979), Adair (1984) and Diaper (1990), in their discussion of reactivity, presuppose subject identification or commitment with regard to experimental procedures. Thus, a reasonable assumption is that the likelihood of a Hawthorne-type reactivity effect, is affected by the length of the study and the number and strength of reactivity-prone factors. In that respect, the designs used in studies reporting remedial effects of VDU-screen filters have not convincingly ruled out the possibility of a Hawthorne-type reactivity effect. The objective was to investigate whether or not VDU-screen filters influence subjective symptoms and other health indicators, and to study the stability of any changes over a longer time interval than commonly used. Further, the objective was to determine if any remedial effects of VDU-screen filters should be attributed to the Hawthorne effect.

2. Method The study, based upon a traditional quasi-experimental simulated control group design (Cook and Campbell, 1979), was conducted in an ordinary office environment. In order to enhance validity, the total study period of five months was split into two parts, 212 months each. The first part of the study was carried out according to a simulated control group design while the second part was a partly switched replication, in which the initial simulated control group acted as a new experimental group. By demonstrating the experimental effect in two populations at two points in time, the validity of the study is enhanced (Cook and Campbell, 1979). The expansion of the design also increases the chance of detecting delayed effects. Two VDU-screen filters, differing in filter surface quality, were used in the two study parts, making it possible to discuss the particular contributions of

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reduced exposures to electrical fields and improved screen legibility. Fig. 1 shows the outline of the study design. 2.1. Research questions The experimental design allows different outcomes to be predicted. If the primary explanatory variable is reduction in electrical fields, a reduction in all or some health parameters could be expected during the initial part of the study in the continuous screen filter (CSF) group, followed by a corresponding reduction in the initial control group (ICG) during the replication part of the study, when this group were provided with screen filters. If health improvement is primarily accounted for by differences in filter surface quality, the effect in the CSF during the first part of the study should differ from the results obtained in the ICG, during the replication period when this group was provided with screen filters. If shortlived subject reactivity such as the Hawthorne effect were the primary contributor, temporary health improvements, especially in subjective measures, would be expected in the CSF group. In the ICG, no effect would be expected during the first part of the study, while a less pronounced effect would be expected during the replication part due to the time lag between study onset and the implementation of screen filters in this group. 2.2. Subjects Participants were 89 office clerks, 8 men and 81 females, from the Oslo branch of The National Insurance Office for Social Insurance in Norway. Mean age was 53 years (SD:11.2). All participants were experienced VDU-users, worked on the same floor and had similar work tasks, mostly data entry and routine data dialogue. VDU use accounted for more than 50% of their working time. For practical reasons, management had organised the employees into ten independent work units of 7–11 persons. Each work unit was randomly assigned to one of the experimental groups (see Fig. 1). The division resulted in only minor differences in age and gender distribution.

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Fig. 1. Outline of the study design.

2.3. Equipment The VDU-equipment used by most subjects was a green-fluorescent monochrome display unit with light characters on dark background (IBM-4704). Any malfunctions were corrected before measurements were taken. Two different VDU-screen filters were used the study: Filter 1, (F1) was a multi-coateded glass filter mounted in a frame which was attached to the VDU’s bezel and grounded with a separate wire attached to the filter frame. Filter 2, (F2) was a micromesh filter mounted directly on the VDU-screen and grounded inside the VDU. Both filters had been tested by independent laboratories and, according to the companies, decrease electrostatic fields by almost 100%. Some reductions in AC-fields were also reported with both filters. No independent testing of shielding properties was conducted as a part of the study. 2.4. Measures Both objective and subjective measures were recorded. Subjective measures were recorded by a modified version of a questionnaire developed in previous studies (Lie and Watten, 1994; Lie and Fostervold, 1995; Fostervold et al., 1995). The

questionnaire was divided in three sections. The first part contained questions about subjective symptoms from various parts of the body. The symptom checklist was comprised of symptoms previously reported by patients of the clinical practice of one of the authors (IL). Subjects were asked to rate the perceived intensity of a given symptom on a Likert-type scale, from 1 (none) to 5 (heavily bothered). A statistical reliability analysis of the symptom list showed a coefficient alpha (Cronbach, 1951) of 0.86, which is almost identical to the 0.88 coefficient alpha level reported for the checklist used by Lie and Watten (1994). Legibility, glare and other ergonomic factors were graded subjectively on a 5-step scale, while psychosocial conditions on the worksite were graded subjectively on a 4-steps scale. Musculoskeletal conditions in the upper part of the body (neck, shoulders and upper back) were evaluated clinically (muscle palpation) by a physiotherapist who was blind with regard to group assignment. Both stiffness and tenderness were recorded on a Likert-type scale, graded from 1 (no stiffness/ tenderness) to 4 (very stiff/tender). Objective measures of productivity and sick leave were also recorded, but were only available as a combined measure for each work unit. Sick leave was recorded as three different measures: long-term

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sick leave (more than three days), short-term sick leave with a doctor’s note, and short-term sick leave with a personal note. (Norwegian legislation gives employees the right to take short-term sick leaves four times a year without consulting a medical practitioner.) The number of people on sick leave and the number of days they were absent were recorded for each category. Productivity was defined negatively as the number of arrears (applications that had not been processed) that each unit transferred from one period to the next.

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rotation was used to create empirical sub-indices from the subjective symptoms part of the questionnaire. Other statistical methods are stated in the results.

3. Results Statistical analyses (independent-samples t-tests) showed no significant differences between the CSF and the ICG at baseline for any measures, including differences based on gender.

2.5. Procedure 3.1. Subjective measures The study was conducted over a five-month period and consisted of two interventions (see Fig. 1). The first intervention consisted of mounting VDU-Screen filter 1 (F1) in the CSF group. The ICG served as a simulated control group during the first part of the study. After a period of 212 months, the second VDU-screen filter (F2) was mounted in the ICG. The CSF group continued with the same filter. The replication part of the study lasted for another 212 months. As shown in Fig. 1, subjective measures and musculoskeletal condition were recorded three times: (1) a baseline measure obtained before randomisation of work units and the first intervention (pre-test), (2) an effects measure before the second intervention (post-test 1), and (3) another effects measure 212 months after the second intervention (post-test 2). Productivity and sick leave were recorded as short time series in order to obtain reliable trend measures. Productivity and sick leave measures were recorded every 14 days. Baseline recordings on these variables began 112 month before the first intervention. 2.6. Statistical methods A two-factor analysis of variance (ANOVA) with one repeated factor (time) and one group factor (VDU-screen filter) was used as the primary statistical method for between subject comparisons. Paired-samples t-test was used as the primary method for within subject analysis. Results were considered significant at p50.05. Factor analysis using principal component analysis with Varimax

The analysis of the subjective measures revealed no significant changes in subjective assessment of psychosocial conditions. However, a tendency toward improvement was present in both groups especially during the initial part of the study. A test–retest analysis of glare and legibility ratings was conducted by correlating the pre-test and post-test1 scores in the ICG. The result showed a stability coefficient (Pearson’s r) of 0.89, indicating that the legibility and glare ratings were stable for the ICG during the first part of the study. Results from the first part of the study showed a 15% improvement in subjective ratings of legibility and glare in the CSF and a clinically insignificant 2% improvement in the ICG. This is comparable to the screen-filter induced legibility/glare improvements found by Collins et al. (1990), but somewhat smaller than those found by Hedge et al. (1996). During the second part of the study, the ICG, now using screen filters, improved its legibility/glare ratings by 11% while the CSF group, who at this point had used screen filters for 5 months, reduced their evaluation by 9%. Thus, only a 6% improvement was maintained throughout the whole study period. None of these changes was, however, statistically significant. Nor were there any statistically significant changes in any of the other ergonomic variables. A summary table of psychosocial, glare/legibility and ergonomic measures is shown in Table 1. In order to evaluate the stability of symptoms over time, a test–retest analysis was conducted on the symptom scores obtained at pre-test and the

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Table 1 Summaries of pre-test, post-test 1 and post-test 2 scores for the CSF group and the ICG groupa Continuous screen filter group Pre-test

Psychosocial Ergonomic Glare/legibility

Initial control group

Post 1

Post 2

Pre-test

Post 1

Post 2

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

2.80 2.25 2.16

0.49 0.74 0.71

3.00 1.97 1.84

0.32 0.73 0.78

2.98 1.99 1.96

0.47 0.66 0.63

2.88 2.37 2.36

0.37 0.83 0.77

2.94 2.39 2.33

0.44 0.75 0.72

2.89 2.14 2.05

0.49 0.76 0.67

a Subjectively reported ratings of psychosocial condition at the workplace, ergonomic condition, glare and legibility. M=arithmetic mean, SD=standard deviation. Psychosocial condition at the workplace improves with increasing numbers. Ergonomic condition, glare and legibility deteriorate with increasing numbers.

first post-test in the ICG. The stability coefficient (Pearson’s r) was 0.87, indicating relatively high stability of the symptom scores. A decline in perceived symptom intensity was observed in both groups during the initial part of the study. When calculated as an aggregated symptom score, this decline was larger for the CSF, compared to the ICG (F(73.1)=5.21, p=0.025). A two-factor ANOVA with repeated measures showed a significant group-against-time development for subjective symptom reports in the second part of the study (F(59.1)=5.45, p=0.023). No statistically significant symptom decline was found in the ICG after the introduction of screen filters, while a significant symptom aggravation was uncovered in the CSF group (t(27)=ÿ2.57, p=0.016). The group-against-time development for aggregated symptom scores is shown in Fig. 2. Comparisons between groups (independentsamples t-tests) revealed no significant differences between the two conditions, either at the posttest 2, or over the entire study period. However, aggregated symptom scores represent coarse overall measures that can disguise real effects in symptom subgroups. A factor analysis was therefore conducted in order to establish empirical symptom sub-indices. A 4-factor solution explaining 47.4% of the variation was selected based on its ease of interpretation and relatively high commonalties of the variables. Variables were summed to form the following four symptom indices: (1) Ocular symptoms, (2) Musculoskeletal symptoms, (3) General fatigue, (4) Vague feelings of discomfort. A two-factor ANOVA with repeated

measures revealed the same pattern, as mentioned above, to be present in all four indices. No other effects were revealed by the analyses. A summary table of the four symptom indices, at pre-test, post-test 1 and post-test 2 are shown in Table 2.

3.2. Musculoskeletal condition Musculoskeletal conditions were analysed at different body specificities. Tenderness and stiffness from the left and right side of the upper back and shoulders were analysed separately before the aggregated tenderness/stiffness scores from each part of the upper body. Three new aggregated measures were then subsequently calculated: musculoskeletal stiffness, musculoskeletal tenderness and a welded musculoskeletal measure. The main results (independent-samples t-tests) show that no significant differences were present between the CSF and the ICG at either post-test 1 or posttest 2. The two-factor ANOVA with repeated measures did not reveal any group-against-time interaction, indicating that the eventual pre/post 1/ post 2 changes over time were independent of group assignment. The group-against-time development for the welded musculoskeletal measure is shown in Fig. 3. Although no significant changes were observed during the study period, the pattern shown in Fig. 3 is worth mentioning. Both groups show a decline in symptom intensity during the first intervention period, followed by an increase during the second period. Even though not

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Fig. 2. Subjective symptom report, aggregated scores. Group-against-time development.

Table 2 Summaries of pre-test, post-test 1 and post-test 2 scores for the CSF group and the ICGa Continuous screen filter group Pre-test

Ocular symptoms Musculoskeletal General fatigue Discomfort

Post 1

Initial control group Post 2

Pre-test

Post 1

Post 2

M

SD

M

SD

M

SD

M

SD

M

SD

M

SD

1.65 2.67 2.19 2.22

0.55 0.69 0.56 0.67

1.41 2.27 1.92 1.86

0.48 0.66 0.58 0.62

1.55 2.51 2.05 1.97

0.48 0.81 0.65 0.67

1.79 2.78 2.12 2.10

0.69 0.77 0.63 0.65

1.80 2.68 2.07 2.06

0.82 0.69 0.69 0.73

1.61 2.76 2.10 2.00

0.72 0.81 0.69 0.67

a

Subjective scores on four empirically created symptom indices: 1. Ocular symptoms, 2. Musculoskeletal symptoms, 3. General fatigue, 4. Vague feelings of discomfort. M=arithmetic mean, SD=standard deviation. Symptom severity increases with increasing numbers.

statistically significant, the tendency is comparable to the pattern found for subjective symptoms. 3.3. Sick leave Short-term sick leave, and especially short-term sick leave with a personal note, is considered by many medical practitioners and ergonomists in Norway to be a relatively sensitive indicator of work-induced discomfort. Special attention was therefore directed toward these measures during the statistical analyses. The results show large variations in sick leave for both groups. Some of

these variations are likely due to seasonal variability, while others may be caused by unknown factors. The sick leave measures were calculated on three different levels of specificity with regard to type: (1) the total percentage of sick leave during a specific period, (2) short-time sick leave, and (3) short-time sick leave with a personal note. This provides six different sick-leave indices, since both the number of days absent and the percentage of people absent in each work unit were analysed for each specificity level. The group-against-time development for the percentage of people on short-time sick leave with a personal note in

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Fig. 3. Musculosceletal condition, welded measure. Group against time development.

Fig. 4. The percentage of people on short-time sick leave with a personal note in proportion to the total number of people on sick leave. Group-against-time development.

proportion to the total number of people on sick leave is shown in Fig. 4. Fig. 4 indicates that both groups followed the same general developmental course throughout the

study. With some variations, this was also the case for the other sick-leave indices. While the statistical analysis revealed no significant groupagainst-time differences, some remarks should be

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made regarding the sudden drop in sick leave found in the CSF group during the first part of the study. Corresponding declines in sick leave followed by increases were also found in the other sick leave indices. That indicates that a brief decline in sickness was present in the CSF group during the first part of the study. 3.4. Productivity Productivity results show essentially the same pattern as measures of sick leave. Both groups had approximately the same developmental course, and the statistical analysis revealed that no significant differences were present between the two groups. The group-against-time development of arrears during the study is presented in Fig. 5. Again, there is (see Fig. 5) a reduction in the number of arrears during the first part of the study, followed by an increase. In contrast to measures of sick leave, the decline in arrears is almost identical for both groups.

4. Discussion This study investigated the relationship between various health indicators and the use of VDU-

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screen filters. Three different factors were proposed as explanatory variables: (1) electrical shielding properties, (2) VDU-filter surface quality, and (3) subjective reactivity, e.g., the Hawthorne effect. The study design allowed three different outcomes to be predicted, depending upon the primary explanatory variable. For most health parameters, the first impression would be that the results do not support the remedial effect of VDU-screen filters. There is, however, an obvious contrast between the two study parts. When the first part of the study is considered independently of the second part, a significant difference is present in subjective symptom reports. Based solely on this finding, a plausible conclusion would be that VDU screen filters have a positive effect on VDU-workers’ well being. The same conclusions are made in studies using short treatment intervals (Oftedal et al., 1995; Hedge et al., 1996; Hladky and Prochazka, 1998). However, the positive trend observed for subjective symptoms in the CSF group during the first part of the study was followed by a negative trend during the second part. In addition, only minor changes were observed in the ICG after they were provided with VDU-filters. Thus, the CSF group showed an improvement in subjective symptoms, followed by a decline, while the ICG

Fig. 5. Number of arrears. Group-against-time development.

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remained relatively stable. Slight trends, similar to the pattern observed for subjective symptoms, were also observed for musculoskeletal measures, sick leave and productivity. The results seem, therefore, not to be consistent with the expectations made when electrical shielding properties are the primary explanatory variable. An alternative explanation is the differences between the two types of screen filter. Both filters have the same electrical shielding properties, leaving differences in filter surface material as their most important distinction. Bergqvist and Wibom (1993) considered the reduced screen luminance and edge sharpness experienced when using micromesh filters to be a possible explanation for their findings of increased aches and discomfort associated with VDU-filters. Differences in quality between the glass filter (F1) and the micromesh filter (F2), with regard to legibility and glare, could therefore explain the lack of health improvement found in the ICG during the second part of the study. However, this does not explain the aggravation observed in the CSF group during the second part. Moreover, the subjective ratings of legibility and glare do not indicate large differences in quality between the two types of filter. Thus, considering both study parts, the results are not explained by filter quality. Another possibility is that increased work-pressure at the work-site, alone or in conjunction with filter properties, accounted for both the lack of improvement found in the ICG and the aggravation found in the CSF group during the second part. However, subjective ratings of the psychosocial and general ergonomic conditions at the work-site do not indicate that this was the case. A comment should also be made to the high proportion of females in the sample. It is known from other studies that women tend to report higher incidences of symptoms and use of medical care services than men (Korunka et al., 1999; Green and Pope, 1999). However, no gender-related differences were present in this study and Korunka et al. (1995) reported no interaction between gender, psychosomatic complaints and the implementation of new technologies (VDUs). Our conclusion is therefore that the cause of the observed symptom pattern is not directly linked

to the use of VDU-screen filters. Hence, it follows that the nexus of cause and effect are unclear and that some factor(s) other than the treatment variable caused the reduced symptomatology found in the continuous screen filter group during the first part of the study. As matters now stand, it is appropriate to turn to the expectations made when the Hawthorne effect is seen as the primary explanatory variable. As usual in field research, no effort was made to conceal the treatment variable. Neither was there introduced any bogus or placebo treatment in the ICG during the first part of the study. It was not possible to restrict inter-subject contact during the experiment and all subjects knew the purpose of the study and the dependent measures. Although the possibility of biasing the measurements, in terms of subjective reactivity, varied over the different measures, modifications of the dependent measures required changes in personal behaviour. Moreover, none of these behaviours involved tasks that could be characterised as well practiced skills. Participants assigned to the CSF were fully aware of the purpose of the study, the independent variables and the expected outcome. Subjects assigned to the ICG were likewise aware that they were members of the control group, receiving no treatment during the first part of the study. Finally, it is plausible to interpret the improvement in psychosocial conditions at the work site as an indication of subject commitment to the study. In brief, factors that have shown to result in a Hawthorne-type reactivity effect were ostensibly present during the study. With respect to the previous discussion of reactivity-prone factors, a Hawthorne effect was expected, especially on subjective symptom reports, in the continuous screen filter group, but not in the initial control group during the first part of the study. The results are in accordance with this expectation. On the other hand, if a placebo treatment had been introduced in the control group during the first part of the study, creating similar excitement in both groups, reactivity would be presumable in this group also. This condition represents a possible explanation for the findings reported by Oftedal et al. (1995). In their study, VDU-screen filters with disabled grounding were

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provided to the control group. Despite the absence of shielding properties, the effect was almost as large in the control group as in the experimental group. Generally, the Hawthorne effect is considered temporary, although it is difficult to estimate its active period. It is reasonable to assume that this is a function of habituation. With time people adapt to the treatment variable. Consequently, a drop in excitement and commitment reduces the power of the Hawthorne effect. In this respect, the decline over time represents a plausible explanation for the aggravation found in the CSF group during the second part of the study. At the same time, it explains the lack of effect of the screenfilters on the ICG during the same period. Although they were not using screen filters themselves, working among colleagues who were using screen filters for 212 months, most certainly accustomed subjects in the ICG to the treatment variable. Screen filters had become commonplace in the workplace. This ostensibly affected the enthusiasm and commitment, felt by subjects in the ICG even after they received screen filters. With respect to the previous discussion, the results obtained in the second part of the study may be explained in terms of the predictions made when reactivity factors associated with the Hawthorne effect are regarded as the primary explanatory variable. In conclusion, the results showing reduced symptomatology in the CSF group, in the first part of the study, are eclipsed by the results of the study as a whole. The results do not support the use of VDU-screen filters as an effective remedy for reducing work-related healthproblems among VDU-workers. When regarding a Hawthorne-type reactivity effect as the primary explanatory variable, the results for both groups, in both parts of the study, were in accordance with expectations. This study demonstrates that conclusions drawn from short-term intervention studies might be premature if caution against reactivity-prone factors is not taken. It emphasises the need for a better understanding of subject expectations, and stresses the importance of designing field studies to accommodate for the possibility of psychological

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and task-related factors that convey situational meaning to subjects.

Acknowledgements We wish to thank Prof. Svein Magnussen, Dag Erik Eilertsen and Dennis Ankrum for valuable help and comments on the manuscript.

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