5. Statistical regression. A person's performance of any task varies within a certain range. On the average, you may be able to do 10 pull-ups, but on a particular day you may do 8, 9, 11, or 12. In fact, there may be days when your performance is quite unusual--you can barely do 5 pull-ups, or somehow you manage to do 18. However, if you did pull-ups the next day, and the day after that, your performance would probably regress (move) to the mean, or your average performance.
In research, a group's pre-test performance might (by chance) be unusually high or low; some people had a good day or a bad day. On later testing, the group's performance regresses to the mean (i.e., is more usual). The researcher may mistakenly treat differences between pre- and post-test scores as the result of an intervention ("They improved.") or as the failure of an intervention ("They got worse!"), when in fact, the group merely turned in its average or usual performance.
The rival hypothesis of statistical regression can be partly controlled by using equivalent comparison groups, since the possibility of unusual scores applies equally to the groups.