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.