Selection Bias in Occupational Sperm Studies

Abstract

Participation rates in sperm studies are typically 25-50%, and therefore it is a matter of concern whether the men who provide semen samples truly reflect the parent population. The authors analyzed data from three Danish occupational sperm studies and evaluated the relation between age and occurrence of subfertility to participation and provision of semen samples. Age and subfertility were found to be rather strong determinants of participation. Willingness to provide semen samples was greater among men aged <40 years (odds ratio (OR) = 1.8, 95% confidence interval (CI) 1.3-2.6) and among men who had experienced an infertile period (OR = 1.7, 95% CI 1.3-2.1). Furthermore, the effect of infertility was modified by occupational exposure status, thus resulting in a tendency to differential selection and possibly biased risk estimates. In a cross-sectional study, the authors recommend that priority be given to a high participation rate and that data on the basic variables be collected from the entire study population, so that it is possible to make a nonresponder analysis and evaluate bias. Such an analysis can be carried out by means of questions enquiring about infertility, genital disorders, and earlier seminal examinations. Although the longitudinal study design is without many of the limitations of the cross-sectional study, a longitudinal study is usually not feasible.

Change in semen quality and sperm chromatin structure following occupational styrene exposure

“…It is our impression that at least some of the dropouts participated only to obtain a free infertility checkup. Although we observed no indication of dropout conditional to the styrene exposure level, this clearly illustrates the potential for selection bias in cross-sectional studies (Larsen et al 1998a). To avoid selection bias on an individual level the present analysis was therefore restricted to the 23 workers who provided semen samples both before and during styrene exposure.…”

Glycol ethers and semen quality: a cross-sectional study among male workers in the Paris Municipality

“…25 Ideally, to identify a selection bias, basic information should be collected from the entire study population and a non-responder analysis undertaken. 26 Unfortunately, in our study we managed only to obtain basic data from 34% of men who did not want to participate.…”

Measuring male infertility: epidemiological aspects

“…The first group oversamples those with problems, and these forces of selection have most likely changed over time. Furthermore, studies indicate that males who volunteer for semen stud-ies oversample those with perceived fertility problems 4,5,36,37 . Other biomarkers, including inhibin B, that only require blood sampling are of interest.…”