ну вот тебе по-простому (http://jonoscript.wordpress.com/2010/10/09/test-pilot-self-selection-bias-and-how-to-compensate-for-it/):

"Self-selection bias is the bias that creeps into your results when the subjects of your study are people who choose to be subjects. It’s a bias because a group of people who choose to be subjects is not the same as a random sample of the population.