Introduction

Since the end of the 1980s, the Dutch government has been attempting to promote participation in employment, among other things to spread the financial burdens of health care, social welfare and pension schemes (that will increase due to ageing of the population) over a larger number of people with paid employment (Kerkhofs et al. 2000). This policy is expressed for example in the recent plans to increase the retirement age by constraining pension regulations. In addition, it has become much more difficult to be declared disabled and it is strived for that as many people as possible who are receiving unemployment benefits or disability pension will find jobs in the short-term (Wevers et al. 1996). If this government policy succeeds, it will mean that a large proportion of workers have non-optimal health. This not only applies to the 60-year-olds who cannot take early retirement and to the people who have been excluded from receiving disability pension, but also to newcomers to the labour market: research has shown that a larger proportion of unemployed people suffer from health problems than people with paid jobs (Wetenschappelijke Raad voor het Regeringsbeleid 2001). Frequently, adjustments in the work situation will be necessary in order to make it possible for employees with non-optimal health to function adequately.

Until now, very little research has been performed into the question of how employees with ill health perceive their work. Do they experience the same problems as their healthy colleagues? or do they complain about more or different aspects of their work? It is necessary to gain more insight into work-related characteristics that might cause problems, to help advise employers about desired adjustments to the working environment or workload.

If the burden from the work or from work-home interference becomes too high, this can lead to health effects, such as fatigue, emotional exhaustion and perceived health complaints (Van Dijk et al. 1990; Donders et al. 2003). Many employees complain of fatigue. Prevalence rates of chronic fatigue in the general population vary widely: depending on the definition and measurement method used, they vary from 7 to 45% (Lewis and Wessely 1992). In a large cohort study on the working population in the south of the Netherlands, a recent prevalence of 22% ‘long-term fatigue’ has been established (Kant et al. 2003). In the case of emotional exhaustion, a central symptom of burnout, the feeling of being mentally and emotionally ‘spent’ was found to be correlated with chronic overburdening at work (De Ridder et al. 2000). Besides fatigue and emotional exhaustion, a person’s perceived health in broader terms serves as a relevant means to measure health status. Perceived health complaints appeared to be associated with work-related characteristics (Karasek 1979; Donders et al. 2003; Sluiter et al. 2003) and absenteeism, both in chronically ill and non-chronically ill workers (Roskes et al. 2004).

Resent research has shown that chronically ill workers have higher levels of fatigue (Franssen et al. 2003; Boot 2004) and absenteeism than their healthy colleagues (Kessler et al. 2001; Baanders et al. 2002). Fatigue was often found to be a concomitant and very invalidating symptom of the chronic disease concerned (Swain 2000; Franssen et al. 2003; Weijman et al. 2003). By correcting for chronic disease, such as for example in the study on fatigue by Bültmann et al. (2002b), meaningful differences between the chronically ill and their healthy counterparts may have been missed. Possible differences in associations between work-related characteristics and health effects will be more easily identifiable if separate analyses are performed on the two groups.

In this study, the work perceptions of workers with chronic diseases were compared to the perceptions of workers without chronic diseases. Data were available from a questionnaire survey on work and health in employees at a Dutch university. Differences in scores on fatigue, emotional exhaustion, perceived health complaints and work-related characteristics were investigated between these two groups. In addition, data from the chronically ill workers (CIWs) and the non-chronically ill workers (NCIWs) were analysed separately to see whether there were differences in associations between work-related characteristics and the three health-related characteristics in the two groups.

Methods

Data collection

Data for the present study were obtained via a questionnaire mailed to the home addresses of all the workers at a Dutch university. By means of an accompanying booklet, the employees were informed about the objectives of the study and assured that their responses would be handled with confidentiality. After 3 weeks, all the workers received a reminder. Both the University Board and Works Council of the university approved the study. Demographic characteristics that have been mentioned as potential confounders in the literature (e.g. sex, age, employment function, hours worked weekly) were included in the questionnaire (Van de Mheen et al. 1999; McDonough and Walters 2001; Kessler et al. 2001; Molarius and Janson 2002), as well as chronic disease, work- and health-related characteristics.

Demographic characteristics

Sex was coded as 0 = male and 1 = female. Age was divided into four categories: (1) <36 years, (2) 36–45 years, (3) 46–55 years and (4) >55 years. Education level was categorised into three categories: primary school/lower vocational education, secondary/higher vocational education or university. Based on occupation and education level, employment function was categorised as: lower-educated non-scientific personnel (1 = low NSP), higher-educated non-scientific personnel (level of education is college or university degree; 2 = high NSP) or scientific personnel (3 = SP). Total numbers of hours worked weekly (all jobs together, including overtime) were categorised as: (1) <25 h, (2) 25–40 h, (3) >40 h.

Chronic disease

In response to the question ‘do you have a chronic disease?’ the respondents could choose from the following options: no, diabetes, psychological disorder (depression, anxiety, addiction), cancer, cardiovascular disease, respiratory disease (asthma, bronchitis, COPD), neurological disease (Parkinson, multiple sclerosis, etc.), musculoskeletal complaints (rheumatic arthritis, osteo-arthritis, etc.), or ‘other, namely...’. These options correspond with the chronic diseases most prevalent in the Netherlands (Van den Bos et al. 2000). Those respondents who gave a positive answer to one or more of these chronic diseases were categorised as chronically ill workers (CIWs). Those respondents who marked the option ‘other, namely...’ were also considered to be chronically ill when their disease fell within one of the following newly formed categories: skin disease or gastro-intestinal disease. The remaining respondents were considered individually and categorised as CIWs when they were indeed found to have a disease in the category ‘other, namely...’. Any respondents who reported a complaint that could not be assumed to be a chronic disease (e.g. often having a cold) or who simply did not answer the question on chronic disease were excluded from the analyses (n = 52). All the other respondents were categorised as non-chronically ill workers (NCIWs).

Health-related characteristics

Fatigue (4 items) was assessed using the Shortened Fatigue Questionnaire (Alberts et al. 1997; Boot 2004). Emotional exhaustion (4 items) was measured using the Dutch version of the Maslach Burnout Inventory (De Jonge et al. 2000). Perceived health complaints were measured using the VOEG-13 that includes 13 dichotomous items to assess whether a person occasionally suffers from a range of psychosomatic complaints (Dirken 1969; Geurts et al. 1999).

Work-related characteristics

Work-related characteristics were assessed using various parts of previously validated questionnaires. Work variety (3 items) and professional expertise (2 items) were measured using the Maastricht Risk Assessment Questionnaire (De Jonge 1994; De Jonge et al. 2000). Work pressure (8 items) and autonomy (10 items) were measured using the Maastricht Autonomy Questionnaire (De Jonge et al. 1995). Role conflict (5 items), role ambiguity (5 items), social support from superiors (5 items) and social support from colleagues (5 items) were measured using the Questionnaire Organizational Stress-D (Bergers et al. 1986). Information on work (3 items), communication (4 items), possibilities for learning (4 items), decision latitude (9 items) and physical workload (3 items) were assessed using the Questionnaire on Experience and Evaluation of Work (Van Veldhoven and Meijman 1994; Van Veldhoven and Broersen 1999). Career opportunities (3 items) and employment terms (5 items) were also measured (De Jonge et al. 2000; Donders et al. 2003).

Analyses

The small number of respondents with missing data (mean percentage 1.7%) on health- and/or work-related items were assigned the mean item score for the corresponding category of employment function. Item scores on fatigue and perceived health complaints were summed for every respondent and mean item scores were calculated for emotional exhaustion and all the work-related characteristics. Cronbach’s alphas were calculated for each of the health- and work-related characteristics. A Cronbach’s alpha of 0.70 or higher was taken to reflect good internal consistency (Nunnally 1978). Cronbach’s alphas were high for the health-related characteristics (>0.75). Most of the work-related characteristics also had high Cronbach’s alphas (>0.70), with the exception of work variety (0.67), career opportunities (0.68) and employment terms (0.63).

Differences between CIWs and NCIWs

Demographic data of the CIWs and NCIWs were compared using the χ 2-test. Mean scores of the CIWs and NCIWs on the various health- and work-related characteristics were compared using the general linear model (GLM). Sex, age, employment function and hours worked weekly were included as ‘fixed factors’, in order to adjust for potential confounding influences (Van de Mheen et al. 1999; McDonough and Walters 2001; Kessler et al. 2001; Molarius and Janson 2002). The significance level in all the tests was set at P ≤ 0.05.

Associations between work- and health-related characteristics

Linear regression (enter method) was used to investigate the degree to which the different work-related characteristics contributed to explaining the variation in emotional exhaustion, fatigue and perceived health complaints. Analyses were performed on the total population, with presence of chronic disease as variable in the model. In addition, separate analyses were preformed on the CIWs and NCIWs to investigate where differences were present between the two groups. If the regression coefficient in one group was clearly higher than that in the other group, then an interaction term was made between presence of chronic disease and the relevant work-related characteristic by multiplying the variables with each other. Subsequently, regression models were calculated that also included the relevant interaction terms. The interaction terms between presence of chronic disease and the potential confounders sex, age, employment function and hours worked weekly, were included in each model, irrespective of whether there were differences between the two groups.

All the analyses were conducted in SPSS 12.0.1.

Results

Response rate and characteristics of the population

A total of 3,881 questionnaires was distributed and 1,843 were returned, which yielded a crude response rate of 47.5%. However, 16 people indicated the receipt of two questionnaires because they had more than one job at the university, 41 questionnaires were returned as undeliverable and 73 individuals were no longer employed at the university anymore. The adjusted response rate was 49.1%. Compared to the sex distribution in a personnel database, fewer men than women responded (χ 2 = 71.8, P < 0001). Analyses were performed on 444 CIWs (24.8%) and 1,347 NCIWs (75.2%).

The distribution of chronic disease among the CIWs was as follows: musculoskeletal complaints (33.3%), respiratory diseases (22.1%), psychological disorders (11.7%), cardiovascular diseases (10.1%), diabetes (6.1%), cancer (4.3%), neurological diseases (3.6%), gastro-intestinal diseases (3.2%), skin diseases (3.2%) and other (e.g. allergies, migraine, diseases of the thyroid gland, etc.) (19.4%). Comorbidity was present in 13.3% of the CIWs.

Significant differences in age, education level, employment function and hours worked weekly were found between the CIWs and NCIWs, but not in sex (Table 1). On average, the CIWs were older than the NCIWs. A higher percentage of the CIWs also had a lower education level and a low NSP function than the NCIWs. Compared to the NCIWs, more of the CIWs worked for <25 h a week, while fewer of them worked for >40 h a week.

Table 1 Demographic characteristics of the study population in percentages
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Work- and health-related characteristics in CIWs and NCIWs

The CIWs reported significantly higher scores on fatigue, emotional exhaustion and perceived health complaints than NCIWs (Table 2). In addition, the scores of CIWs on work-related characteristics in general were less favourable than those of NCIWs. The CIWs had significantly higher scores than the NCIWs on the negative work-related characteristics physical workload and role conflict. Regarding the positive work-related characteristics, the scores of the CIWs were significantly lower, especially on social support from superiors and colleagues, decision latitude and career opportunities. The associations between the work-related characteristics and the health-related characteristics are shown in Table 3.

Table 2 Mean scores and standard deviations (SD) for the health- and work-related aspects (adjusted for sex, age, employment function and hours worked weekly)
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Table 3 Standardised regression coefficients (β) and proportions of explained variance (R²) in the total group (Total), non-chronically ill workers (NCIWs, n = 1,347) and chronically ill workers (CIWs, n = 444)
Full size table

Chronic disease

Presence of chronic disease was included as variable in the model that contained the total population. The results showed that the presence of chronic disease was positively associated with each of the three health-related characteristics (Table 3). The association with emotional exhaustion was less strong than with perceived health complaints, while the association with fatigue lay between these.

Fatigue

In the total population, higher scores on the negative characteristics role conflict, work pressure, role ambiguity and physical workload were associated with more fatigue. Greater satisfaction with employment terms, work variety, information on work, possibilities for learning and support from colleagues were associated with less fatigue.

In the separate analyses on the CIWs and NCIWs, role conflict, work pressure, role ambiguity, physical workload, employment terms, information on work and social support from colleagues played a part in the NCIWs, but not in the CIWs. Unpleasant treatment and autonomy played a part in the CIWs, but not in the NCIWs. The difference in unpleasant treatment between the CIWs and NCIWs was significant: this interaction term was retained in the last regression model. This means that in the CIWs, unpleasant treatment played a more important part than in the NCIWs. In the NCIWs, the work-related characteristics explained a larger proportion of the variance in fatigue than in the CIWs (24 vs. 16%).

Emotional exhaustion

More role conflict, work pressure, role ambiguity and physical workload were associated with more emotional exhaustion. Particularly work pressure played a large part (β = 0.32). Employment terms, work variety, possibilities for learning, social support from superiors and from colleagues were negatively associated with emotional exhaustion, which means that a higher score on these work-related characteristics was related with less emotional exhaustion. More autonomy, however, was associated with more emotional exhaustion.

In the separate analyses, differences were found in unpleasant treatment, role conflict, physical workload, communication, possibilities for learning, social support from superiors and from colleagues between the NCIWs and CIWs. On the basis of the interaction terms, it appeared that unpleasant treatment in the CIWs was more strongly associated with emotional exhaustion, whereas in the NCIWs, there was a stronger association with physical workload. The other differences were not statistically significant. Percentages of explained variance were comparable between the NCIWs and CIWs: 37 and 36%, respectively.

Perceived health complaints

Once again, associations were found with negative work-related characteristics: more unpleasant treatment, role conflict, work pressure, role ambiguity and physical workload were associated with more perceived health complaints. In the total population, the positive work-related characteristics work variety and information on work played a part. In addition, it was found that more social support from colleagues was associated with fewer perceived health complaints.

Differences between the NCIWs and CIWs occurred in unpleasant treatment, role conflict, work pressure, physical workload, work variety and social support from colleagues. The interaction model showed that this difference was only significant for unpleasant treatment: the association was stronger in the CIWs than in the NCIWs. Percentages of explained variance were slightly higher in the CIWs: 28 versus 20%.

Sex, age, employment function and hours worked weekly

Sex, age, employment function and hours worked weekly were included as potential confounders. Table 3 shows that women experienced more fatigue and health complaints than men. This applied to the CIWs and NCIWs. Younger workers in the NCIW group reported more fatigue and emotional exhaustion than the older workers. In the case of fatigue, the interaction term between presence of chronic disease and age remained in the model, which means that the association between age and fatigue was stronger in the NCIWs than in the CIWs. In the total population and in the NCIWs, people with a higher-ranking job reported more fatigue and exhaustion than workers with lower-ranking jobs. A negative association was found between hours worked weekly and fatigue in the total population and in the NCIWs: more hours worked weekly was associated with less fatigue.

Discussion

This study investigated whether there were differences in scores on the health-related characteristics fatigue, emotional exhaustion and perceived health complaints and various work-related characteristics between workers with chronic diseases and those without chronic diseases. In addition, it was investigated whether there were associations between work-related characteristics and health-related characteristics and whether there were differences between the CIWs and NCIWs.

Comparison of the two groups showed that on average, the CIWs were older, had a lower education level, lower-ranking jobs and more of them were employed part-time than NCIWs (Table 1). These findings support those from other research (Merens et al. 2000; Franssen et al. 2003; CBS 2003a). Generally, it has been found that more women have chronic diseases, but in the present study, this difference was very small.

After correction for sex, age, employment function and hours worked weekly, CIWs had fewer positive work-related characteristics in their work and more negative characteristics (Table 2). Merens et al. (2000) reported the same on the basis of national rates. They remarked that it remained unclear whether CIWs did indeed have poorer jobs (which was possible in view of their lower education level and lower employment status) or whether it was chiefly a perception effect in which the work was experienced as heavier because the people in question were feeling more vulnerable. In our study group, it is most likely that particularly the latter played a part. The CIWs and NCIWs were working for the same employer and it is improbable that this employer was creating better working conditions for NCIWs than for CIWs.

Associations between work- and health-related characteristics

The regression model with interaction terms showed that there were significant differences between NCIWs and CIWs regarding unpleasant treatment and in the association between physical workload and fatigue. It was striking that in each of the three health-related characteristics, unpleasant treatment came forward as a bottleneck in the CIWs. Differences in the association between physical workload and fatigue between the two groups were not very relevant: in the CIWs there was no association, whereas in the NCIWs, the association was significant but weak (β = 0.08).

Separate regression analyses on the two groups showed that in the NCIWs, more work-related characteristics had a significant association than in the CIWs. However, the NCIW group was much larger than the CIW group (n = 1,347 and 444, respectively), which means that regression coefficients will pass the significance level more quickly in the NCIWs. This occurred for example in the association between work variety and emotional exhaustion. In the two groups, β was 0.08, but in the CIWs, this association was not significant. When regression coefficients of larger than 0.10 (Cohen 1977) were taken into account, it was found that particularly the negative work-related characteristics contributed to explaining the variance in the three health-related characteristics.

In the two groups, support from superiors was associated with less emotional exhaustion, but there were indications that the role of social support from superiors was more important in the CIWs than in the NCIWs. Detaille et al. (2003) also reported that social support from superiors was an important factor in a study on people with chronic diseases. Work adjustments are crucial in order to retain employment for employees with chronic diseases (Wevers et al. 1996). In most cases, this does not mean that adjustments need to be made to the physical working environment, but instead, changes are needed in the tasks, increasing job control, lowering of the work pace and changes in working hours. The barrier against actually using such facilities is smaller when the employee feels supported by his/her superiors (Wevers et al. 1996; Detaille et al. 2003).

According to theories on work-related stress, sufficient job control is essential to prevent (psychological) stress and health complaints (Karasek 1979; Johnson and Hall 1988). On the basis of these findings, it was expected that decision latitude and autonomy would have a protective influence on health effects (Karasek 1979). However, the results did not show any significant association between decision latitude and health-related characteristics. In a previous study on this population of university employees, Donders et al. (2003) were also unable to demonstrate any association. A probable explanation is that all the employees within the business-culture of a university have sufficient decision latitude. Thus, this aspect did not make any notable contribution to the explained variance. In the total population, autonomy was positively associated with fatigue and emotional exhaustion. A plausible explanation is that people make greater use of possibilities in the area of autonomy owing to their health problems.

In the NCIW group, fatigue was stronger related with role conflict, role ambiguity and physical workload compared with the CIWs. Moreover, stronger associations were found between social support from colleagues and emotional exhaustion and between work variety and perceived health complaints.

Besides differences, several similarities were found between the NCIWs and CIWs: in the two groups, there was an association between work pressure and the three health-related characteristics and between employment terms and emotional exhaustion. The same applied to the association between physical workload and perceived health complaints and to possibilities for learning and emotional exhaustion. The latter two associations were slightly stronger in the CIWs than in the NCIWs.

Percentages of explained variance for fatigue due to work-related characteristics were higher in the NCIWs than in the CIWs. In people with chronic diseases, fatigue depends to a large extent on their diseases (Swain 2000; Sharpe and Wilks 2002; Bensing and van Lindert 2003). People with chronic diseases often experience changes in their daily functioning, such as decreased physical or mental activity (Franssen et al. 2003). This has a negative influence on the general state of health and on psychosocial functioning. In addition, fatigue is often a symptom of the chronic diseases itself (Franssen et al. 2003; Weijman et al. 2003). The part played by work-related characteristics was therefore somewhat smaller in CIWs than in NCIWs.

Emotional exhaustion in the NCIWs and CIWs was explained to an equal degree by work-related characteristics. A probable explanation is that emotional exhaustion (as formulated in the questionnaire) expressly concerned work-related tiredness. Particularly work pressure was associated with a higher score on emotional exhaustion, which confirmed the results of other research (Houtman et al. 2000; Janssen and Nijhuis 2004). Emotional exhaustion can predominantly be considered as mental tiredness. Physical limitations in people with chronic diseases do not necessarily have any influence on this. It was striking that physical workload did play a part in the NCIWs, although the association was rather weak.

Perceived health complaints were explained to a larger extent by work-related characteristics in the CIWs than in the NCIWs. These complaints were assessed using the VOEG-13. The questions in the VOEG-13 address physical complaints that are often labelled as functional or psychosomatic, but can also form part of a physical disorder (Furer et al. 1995). This might partly explain the higher VOEG scores in CIWs. Percentages of explained variance were higher in the CIWs than in NCIWs, which indicated that work-related characteristics, such as unpleasant treatment and physical workload, nevertheless played a part. The largest association, however, was found with physical workload. Owing to their poorer health, CIWs had less capacity and therefore became more easily physically overburdened (Wevers et al. 1996; Merens et al. 2000; Weijman et al. 2003). This might mean that benefit can be gained from improving the working conditions so that despite their poorer health, CIWs can function as well as possible. It is important in this respect to emphasize that people with chronic diseases were somewhat over-represented among the workers with a lower education level and with a lower-educated non-scientific function. Particularly in these groups, the work is often heavier in a physical sense (Merens et al. 2000).

The women had higher average scores on fatigue and perceived health complaints than men, both in CIWs and NCIWs. These findings agree with the results from many other studies that showed more health complaints in women (Donders et al. 2003) and more fatigue than in men (Bültmann et al. 2002b; Bensing and van Lindert 2003; CBS 2003b). In CIWs, fatigue did not appear to be associated with age, whereas in NCIWs, particularly the younger workers seemed to have more fatigue. In general, relationships between age and fatigue are not very clear (Griffiths 2000; Bültmann et al. 2002a).

In the NCIWs, there were indications that workers with higher-ranking jobs experienced greater fatigue and emotional exhaustion. On average, the part played by employment function was not very large. Bültmann et al. (2001) described that it were particularly work-related characteristics and not so much job descriptions that contributed to the explanation of fatigue and psychological complaints. This was confirmed by the present research, because the contributions of (negative) work-related characteristics were greater than employment function.

Methodological considerations

A limitation of the present study was the cross-sectional design. It was assumed that work-related characteristics led to health effects, but as they were measured simultaneously, it was not possible to say anything about the causality or direction of the associations found.

The proportion of employees with chronic diseases in the study population was comparable with the prevalence of self-reported chronic diseases in the Dutch working population (CBS 2001; Kant et al. 2003). Owing to the fact that the data were gathered anonymously, it is impossible to establish whether selective non-response occurred. Our results showed that the CIWs were older and had a lower education level than the NCIWs, concordant with the situation in the Dutch population (CBS 2003a). The education level of our study population of university employees, however, was higher than the average level in the general population, which means that the generisability of the results is only limited.

The information on chronic diseases, demographics, work-related characteristics and the three health-related characteristics were obtained with the aid of a questionnaire that was compiled from (parts of) validated questionnaires. Internal consistency of the scales was generally good. A point of consideration is whether the questions on work-related characteristics were distinguishing enough for CIWs. It is possible that the questions were too general to adequately assess the specific problems chronically ill people experience at work (Detaille et al. 2003).

Misclassification of the workers into the categories chronically ill and non-chronically ill may have affected the results (Kleinbaum et al. 1982) and led to underestimation of the differences. Kessler et al. (2001) reported that recall bias, ignorance or unwillingness to admit to having a chronic disease due to possible stigmatisation may be of influence. However, other literature showed that the reliability of self-reported chronic diseases was moderate. Particularly self-reported cardiovascular diseases and diabetes were in agreement with the professional diagnosis, whereas there was less agreement in the case of musculoskeletal complaints and lung disorders (Heliövaara et al. 1993; Kriegsman et al. 1996). Respondents with a high education level, like many of the participants in this study, reported more in agreement with the medical diagnosis than those with a low education level (Heliövaara et al. 1993). It is unlikely that the CIWs filled in the questionnaire differently from the NCIWs through deviations in involvement in the study, because the questionnaire addressed many subjects and did not lay emphasis on being chronically ill.

In this study, chronic diseases were analysed as one group, without making any distinctions between diagnoses and comorbidity. Analysing differences between various chronic diseases was not possible due to the small numbers of the sub-groups. However, it is possible that in the various diagnostic categories, there were differences in associations between work-related characteristics and health-related characteristics. The study by Detaille et al. (2003) showed that different groups of chronically ill employees were experiencing the same bottlenecks, but their prioritization varied. For example, support from superiors was the most important characteristic in workers with rheumatoid arthritis, whereas this characteristic took third place in workers with diabetes mellitus.

This study focused solely on associations between the three health-related characteristics and psychosocial work-related characteristics. However, it is possible that other factors also play a part, such as factors in private life (e.g. demands at home, leisure time activities), work-family interference and individual characteristics (e.g. health behaviour, coping style) (Bültmann et al. 2002b).

Conclusions

This study showed clear differences in the scores on work- and health-related characteristics between CIWs and NCIWs. In addition, differences were found in the work-related characteristics that contributed to the explanation of fatigue, emotional exhaustion and perceived health complaints. Particularly unpleasant treatment played a larger role in the CIWs than in NCIWs. Fatigue in the CIWs was explained to a smaller extent by work-related characteristics than in the NCIWs, probably because the chronic disease itself had the most influence on this. Perceived health complaints were explained more strongly by work-related characteristics in the CIWs than in the NCIWs. Based on the differences found in this study, we recommend that in future research, distinctions should be made between people with and without chronic diseases. Important differences will be missed by simply correcting for chronic diseases. Within the working population, chronically ill workers should be considered as a group with lower capacity. They will derive extra benefit from attention to the work-related problems that they experience.