Selection bias (and mitigation of it) is a big topic, and worth googling, if you're interested. I recommend reading 'Bad Science' by Ben Goldacre, which is a fu

Selection bias (and mitigation of it) is a big topic, and worth googling, if you're interested. I recommend reading 'Bad Science' by Ben Goldacre, which is a fun read and goes into fascinating detail about various forms of bias in scientific studies and their reporting in the media, and how such problems can be managed.