Open Access Research article

Are risk estimates biased in follow-up studies of psychosocial factors with low base-line participation?

Linda Kaerlev123*, Henrik A Kolstad1, Åse Marie Hansen4, Jane Frølund Thomsen5, Anette Kærgaard1, Reiner Rugulies4, Sigurd Mikkelsen5, Johan Hviid Andersen1, Ole Mors6, Matias B Grynderup1 and Jens Peter Bonde5

Author Affiliations

1 Danish Ramazzini Centre, Department of Occupational Medicine, Aarhus University Hospital and Regional Hospital Herning, Norrebrogade 44, 2C, 8000 Aarhus C, Denmark

2 Research Unit of Clinical Epidemiology, University of Southern Denmark, Sdr. Boulevard 29, Entrance 101, 3rd floor, 5000 Odense C, Denmark

3 Center for National Clinical Databases, South, Odense University Hospital, Sdr. Boulevard 29, Entrance 101, 3rd floor, 5000 Odense C, Denmark

4 National Research Centre for the Work Environment, Lersø Parkallé 105, 2100 Copenhagen OE, Denmark

5 Department of Occupational and Environmental Medicine, Bispebjerg Hospital, Bispebjerg Bakke 23, Entrance 33, 2400 Copenhagen NV, Denmark

6 Centre for Psychiatric Research, Aarhus University Hospital, Risskov, Skovagervej 2, 8240 Risskov, Denmark

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BMC Public Health 2011, 11:539  doi:10.1186/1471-2458-11-539

Published: 8 July 2011



Low participation in population-based follow-up studies addressing psychosocial risk factors may cause biased estimation of health risk but the issue has seldom been examined. We compared risk estimates for selected health outcomes among respondents and the entire source population.


In a Danish cohort study of associations between psychosocial characteristics of the work environment and mental health, the source population of public service workers comprised 10,036 employees in 502 work units of which 4,489 participated (participation rate 45%). Data on the psychosocial work environment were obtained for each work unit by calculating the average of the employee self-reports. The average values were assigned all employees and non-respondent at the work unit. Outcome data on sick leave and prescription of antidepressant medication during the follow-up period (1.4.2007-31.12.2008) was obtained by linkage to national registries.


Respondents differed at baseline from non-respondents by gender, age, employment status, sick leave and hospitalization for affective disorders. However, risk estimates for sick leave and prescription of antidepressant medication, during follow-up, based on the subset of participants, did only differ marginally from risk estimates based upon the entire population.


We found no indications that low participation at baseline distorts the estimates of associations between the work unit level of psychosocial work environment and mental health outcomes during follow-up. These results may not be valid for other exposures or outcomes.

cohort study; health survey; non-response; psychosocial distress; affective disorders