In research, regression is a concern whenever participants are
selected for their exceptionally high (or low) scores. Suppose a clinical
psychologist is examining how a specific treatment influences the social
skills of autistic children. A sample of autistic children is selected
because a preliminary test indicates that they have exceptionally poor
social skills. The psychologist administers the treatment and then once
again measures social skills. Because of the children’s extremely low
scores on social skills at the beginning of the study, it is possible that the
children’s scores improve, not because of the treatment but because their
scores regress toward the mean. In general, statistical regression threatens
the internal validity of a research study because it creates the possibility
that the observed changes in the participants’ scores are caused by
regression instead of by the treatments. Threats to internal validity are
summarized in Table 6.6.