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Summary of lecture by Professor Omar Hasan Kasule Sr. for Year 3 Semester 2 PPSD session on 18th March 2008
Selection bias arises when subjects included in the study differ in a systematic way from those not included. It is due to biological factors, disease ascertainment procedures, or data collection procedures.
Selection bias due to biological factors includes the Neyman fallacy and susceptibility bias. The Neyman fallacy arises when the risk factor is related to prognosis (survival) thus biasing prevalence studies. Susceptibility bias arises when susceptibility to disease is indirectly related to the risk factor.
Selection bias due to disease ascertainment procedures includes differences between the 2 compared groups regarding publicity, exposure, diagnostic, detection, and referral procedures. It also arises due to the Hawthorne and Berkson biases. The Hawthorne self selection bias is also called the healthy worker effect since sick people are not employed or are dismissed. The Berkson fallacy arises due to differential admission of some cases to hospital in proportions such that studies based on the hospital give a wrong picture of disease-exposure relations in the community.
Selection bias during data collection is represented by non-response bias and follow-up bias.
Prevention of selection bias is by avoiding its causes that were mentioned above.
There is no treatment for selection bias once it has occurred. There are no easy methods for adjustment for the effect of selection bias once it has occurred.