||Berkson’s bias occurs in case-control
hospital-based studies when the combination of the exposure and the outcome understudy
increases the rate of admission to hospital. Self-selection bias occurs when there is
difference in characteristics between those who volunteer to participate in the a study
and those who do not. Healthy worker effect occurs when studies are performed in working
place and the disease understudy makes people unavailable. Non-response bias occurs due to
refusal or non-response among study groups.