A<unk>  is a statistical<unk>  of a population where some members of the population are less likely to be included than others. An extreme form of<unk> ng occurs when certain members of the population are totally excluded from the<unk>  (that is, they have zero probability of being selected). For example, a survey of high school students to measure teenage use of illegal drugs will be a<unk>  because it does not include home schooled students or dropouts. A<unk>  is also<unk>  if certain members are underrepresented or overrepresented relative to others in the population. For example, a "man on the street" interview which selects people who walk by a certain location is going to have an over-representation of healthy individuals who are more likely to be out of the home than individuals with a chronic illness.
Biased sample
Block design
Point prevalence
Selectable marker
Cumulative incidence
Dose-response relationship
Number needed to treat
Period prevalence