Self-selection bias is a major problem in research in sociology, psychology, economics and many other social sciences.
[2] 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.
An outcome might be that those who elect to do the preparation course would have achieved higher scores in the actual test anyway.
If the study measures an improvement in absolute test scores due to participation in the preparation course, they may be skewed to show a higher effect.
In particular, self-selection affects evaluation of whether or not a given program has some effect, and complicates interpretation of market research.