People who agree to participate in study may be different in terms of exposure, or other important characteristics than those who don’t participate in studies—may increase or decrease risk estimate depending on factors related to non-response Case and Controls e.g. women who work tend to have more pap smears, so if controls in a case control study choose females who stay at home…then there is bias More test answers on https://studyhippo.com/bias-in-epidemiologic-study-designs/