A new memory pill actually does not work at all. However, a researcher gives it to a sample, and by pure chance, he happens to have gotten a sample of people with above average memory. So, he wrongly...

A new memory pill actually does not work at all.  However, a researcher gives it to a sample, and by pure chance, he happens to have gotten a sample of people with above average memory.  So, he wrongly concludes that it was thepillthat was responsible for their good memory, and he concludes that the pill works.  What has happened?