Academic journal article Scandinavian Journal of Work, Environment & Health

Who Needs Selection Bias?

Academic journal article Scandinavian Journal of Work, Environment & Health

Who Needs Selection Bias?

Article excerpt

Present day low participation rates in certain parts of research is concerning. It is not unusual that half (or even more) of those invited to participate in a research project decline the invitation. Many think this will always lead to selection bias regardless of the type of study in question. That is fortunately not the case. Selec- tion bias is a reason for concern in studies that aim to obtain a representative sample concerning risk factors or outcomes. Those who decline participation will often have a higher risk profile (lower social status, smokers etc) and more chronic disorders. Selection bias of the most serious type is related to the specific hypothesis when the hypothesis is known to participants. Being asked to participate in a study on oral contraceptives (OC) and breast cancer, for example, may be more appealing to women with breast cancer who have used OC. Selection is therefore often of concern in case-con- trol studies. Non-response and selection bias is of less concern in follow-up studies where the outcome is not known at the time of recruitment.

The reason for limited interest in "representative- ness" is that scientific inference addresses a potential cause-effect relation in general, not in a specific popu- lation. Whether an established cause-effect relation is present in a specific population with a given effect estimate is a matter of effect measure modification and the distribution of component causes in that population at that given time period.

Non-response in a follow-up study is expected to affect the structure of the population and influence the confounder distribution, often by producing less confounding because those with health problems and extreme lifestyle factors are more likely to decline par- ticipation. …

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