Abstract
<h3>Background</h3> In epidemiological research, shift work systems are commonly assessed using self-reports. The aim of the current study was to evaluate the validity of generally used questions on shift systems by comparing them to objective data. <h3>Material and methods</h3> We matched the working hour characteristics of 3 preceding months based on the pay-roll based registry data to the questionnaire date in 2014. The data comprised of all (n = 11,052 ) employees in a prospective cohort with a work contract during the questionnaire excluding only on-call workers. 81% (n = 8896) had at least 31 work shifts during the 3 months period. <h3>Results</h3> Using objective data as the gold standard, questions on “shift work with night shifts” and “permanent night work” showed high sensitivity (i.e., the proportion of true shift/night workers that are correctly identified; 96% and 90%) and specificity (the proportion of true non-shift/night workers that are correctly identified, 92% and 97%). Self-reported “regular daywork” showed low sensitivity (73%), but high specificity (99%). “Shift work without night shifts” showed both low sensitivity (62%) and low specificity (87%). In analysis of associations between “shift work without night shifts” and health outcomes in the prospective data, age- and sex-adjusted odds ratios (multivariate logistic regression) were lower for subjective compared to objective assessment (e.g. for fatigue during free-time compared to day work 1.21, 95% CI: 0.78–1.87 versus 1.89, 95% CI: 1.06–3.35). Non-responding (n = 2156) to the questionnaire was not associated to the objective shift system but 55% of the dayworkers had at least one year of earlier shift work experience. <h3>Conclusions</h3> These findings suggest that the validity of self-reported assessment of shift work varies depending on the shift system. Exposure misclassification was most common in self-reported shift work without night shifts and regular day work, contributing to bias towards the null in analyses of health effects.
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