Discussion
The main finding of this systematic review is that people appear less likely to report symptoms of CMD in the context of a population based study rather than in a study of the occupational group to which they belong. This difference was reduced but not fully accounted for by differences between these types of studies, in relation to the quality of the studies. This effect is unlikely to reflect true differences in the level of CMD symptoms experienced, because population studies aim to comprise all sections of the population, including those with chronic health problems, long term disabilities and the unemployed, whilst occupational studies will be subject to the health worker effect
[7]; so one would anticipate the reverse. There are aspects of work which are associated with increased psychological distress (e.g. high demands and low decision latitude
[56]), however, the benefits of employment, over not working, for mental health have consistently been outlined and it is established that the prevalence of CMD is lower in working populations compared to the general population
[57]. There were also unexpected findings within the occupational groups: occupations for which one would expect a higher level of traumatic events, such as the military and police, were not found to have a higher prevalence of CMD than other occupational groups, including academics, teachers, white collar workers and social services staff.
We propose that studies directed at the mental health or “stress levels” of particular occupations may be subjected to a systematic bias, one that is not present in true population studies when participants are not selected purely because they belong to a specific occupation. Numerous occupational studies are actually labelled as studies of “work stress”, which may give rise to a framing effect. There is evidence for strong contextual effects in previous experimental and observational studies
[58],
[59],
[60],
[61], which can be defined as the effect of environmental factors on subjective outcomes, in addition to bias in self-report psychological measures
[62]. The setting in which a questionnaire is completed is likely to influence responses. Responses in the occupational studies may have been biased by a framing effect
[63], with the emphasis on job related questions potentially leading to individuals venting their work frustrations through questionnaires which provide an opportunity to report dissatisfaction. This framing effect may be heightened, or conversely lessened, depending on where the GHQ is embedded within the questionnaire and its positioning relative to other psychosocial measures.
