Occurs when parts of the population are left out in the process of choosing the sample –either by not properly framing the population or the method used to collect the data.
Because the U.S. Census goes “house to house,” homeless people are not represented. Illegal immigrants also avoid being counted. Geographical districts with a lack of coverage tend to be poor. Representatives from wealthy areas typically oppose statistical adjustment of the census.
Historically, clinical trials avoided including women in their studies because of their periods and the chance of pregnancy. This means that medical treatments were not appropriately tested for women. This problem has been recognized and is being addressed.