In experimental design, when comparisons are made of groups that have been selected on the basis of their extreme scores, the posttest means of the groups tend to move toward the mean of the entire population from which the extreme groups were selected. this threat to internal validity is called

Statistical Regression

Explanation:

Internal Validity is a degree of how well a research is carried out. Threats to this include History, Maturation, Testing, Instrumentation and Statistical regression threats.

Statistical regression, or regression to the mean threatens internal validity by choosing the participants  who score the highest or lowest in a group of people to take part in an experiment. An example is choosing the worst scoring or the worst behaving students and testing to see if there are improvements after a period of time.

When a second test is carried out, it biases the research by moving them towards the mean of their original larger group.

is there a year set? the question is incomplete

answer; one person; monarch;