Suppose researchers want to know what the prevalence is in the United States of the type of depression that results from seasonal changes in the amount of daylight (sometimes referred to as seasonal affective disorder). Researchers in Portland, Oregon, collaborated on a study with researchers in Boston, New York, Chicago, and Detroit. Participants in the study are drawn from those five cities. Do you see the sampling bias problem? All these cities are in the northern half of the United States and therefore have a different number of daylight hours than does the southern half. Whatever prevalence rate was calculated would not reflect the entire United States.
Fuse/PunchstockExternal validity