Nübling et al. BMC Public Health 2010, 10:428 http://www.biomedcentral.com/1471-2458/10/428 quantitative demands for all occupational groups - and there are a lot of studies and longitudinal sur- veys reporting this [24]. In this case, demands would indeed be high in the nursing professions and higher than before, but not conspicuously so. 3. Systematic differences in the forms of employ- ment prevalent among the various professions could lead to systematic differences in the way working conditions are evaluated. Examples of such potential systematic distortions or biases would be part-time employment, temporary employment, precarious employment. The third hypothesis was tested in the present study in a subgroup analysis. With respondents grouped according to the frequency of on-call duties and accord- ing to whether they worked full or part-time, a clear relation emerged between the level of demands they reported on the one hand and the frequency of on-call duties and the number of hours worked per week on the other. The mean scale values rose significantly for three of the four scales in the field of demands accord- ing to the number of on-call shifts worked in the pre- vious month. The analysis by weekly working hours, in addition, revealed significantly lower demands for employees not working full time. The positive results for HC in this study are thus probably due in part to the very high proportion of part-time workers in HC in Germany and not to parti- cularly favourable working conditions (in full time work- ers less favourable results are found). In our study 21% worked fewer than 15 hours per week and 48% 15-34 hours in HC; in GNH 6% had fewer than 15 hours per week and 32% worked 15-34 hours. And as shown, these high rates of part-time workers are a fact not only in our survey but typical for the German situation. The results for the scale job insecurity are also inter- esting. Compared with the mean of all occupational groups, the result for HC is about average. GNH, how- ever, as well as General Hospital Care nurses, appears to be disproportionately concerned with this topic. This may be due primarily to a high level of qualitative job insecurity; that is, because of changes in pay scale and contractual conditions, employees in nursing professions may have a heightened fear of not being able to find an equivalent position in the event of unemployment or change of job. This would agree with the findings of the NEXT study [25], which showed that in the nursing professions, qualitative job insecurity (e.g. the risk of being transferred to another position) is more of a con- cern than quantitative job insecurity (the risk of unem- ployment). In the 4 items of the COPSOQ scale “Job Insecurity” quantitative as well as quantitative aspects are addressed. An analysis on item level reveals the Page 10 of 12 biggest difference for general hospital care when com- pared to the overall mean in fact for the qualitative aspect of being transferred to another working place without being considered enough: 12 points difference for this item compared to 7 points for the whole scale and 8 points for the worries of becoming unemployed. For the GNH in this study however, the differences are more equilibrated: 10 points for the transfer-item, 14 points for fear of unemployment and 11 points for the whole scale. Thus, fear of unemployment is seen as a problem in both groups but more in GNH than in gen- eral hospital care, while qualitative aspects of job inse- curity are equally prevalent in both groups and markedly higher than in other professions. Strengths and limitations We are aware of the shortcomings and limitations of mere questionnaire assessments [26]. One point is the general limitation of assessment strategies using one data source only; self-reported data on risk factors and outcomes collected at the same time could lead to the so called “triviality trap” [27,28] or “common method bias"- correlations found could be due to (methodologi- cal) artefacts. The linkage of COPSOQ data with other data sources on psychosocial workload would be desir- able, for example with data from occupational health physicians or enterprise records or experts’ job valua- tion. This is what Kompier calls the “multi-source” assessment [24]. Some scientific projects are going on in this field, e.g. a survey with police officers where “sub- jective” COPSOQ data is matched to “objective” data from an in depth check-up by a physician. But it has also to be said, that only some of the psy- chosocial factors at work are assessable with validated “objective measures” and that, what is assessed “objec- tively”, is often rather a physical response than an aspect of the psychosocial work situation (rather an outcome than a risk factor). We would therefore agree with Kom- pier strengthening the job incumbents as experts for their work situation and concluding that the main dichotomy is not “objective” and “subjective” measure- ment but rather the fact if the criteria of good question- naire development and appropriate statistical analysis are met in self-report studies. The COPSOQ-database was established by collecting data from organizations performing surveys together with the research institute - participants may thus not be representative for the working population in general. In ongoing studies applying the German COPSOQ in population-based representative surveys we will be able to control this potential bias. And - like for the other organisations - a source of potential bias in this survey might be the selection of the HC and GNH facilities based on a pool of facilities