Self-selection bias

In statistics, self-selection bias arises in any situation in which individuals select themselves into a group, causing a biased sample with nonprobability sampling. It is commonly used to describe situations where the characteristics of the people which cause them to select themselves in the group create abnormal or undesirable conditions in the group.

Self-selection bias is a major problem in research in sociology, psychology, economics and many other social sciences. In such fields, a poll suffering from such bias is termed a self-selecting opinion poll or "SLOP". The term is also used in criminology to describe the process by which specific predispositions may lead an offender to choose a criminal career and lifestyle.

While the effects of self-selection bias are closely related to those of selection bias, the problem arises for rather different reasons; thus there may be a purposeful intent on the part of respondents leading to self-selection bias whereas other types of selection bias may arise more inadvertently, possibly as the result of mistakes by those designing any given study.

Explanation
Self-selection makes it difficult to determine causation. For example, one might note significantly higher test scores among those who participate in a test preparation course, and credit the course for the difference. However, due to self-selection, there are a number of differences between the people who chose to take the course and those who chose not to. Those who chose to take the course might have been more hard-working, studious, and dedicated than those who did not, and that difference in dedication may have affected the test scores between the two groups. If that was the case, then it is not meaningful to simply compare the two sets of scores. Due to self-selection, there were other factors affecting the scores than merely the course itself.

Self-selection bias causes problems for research about programs or products. In particular, self-selection makes it difficult to evaluate programs, to determine whether the program has some effect, and makes it difficult to do market research.