Jose is a researcher who needs to obtain a sample that "looks like" (is representative of) the population. His population is 65% female and 35% male. He decides that the easiest way to obtain a sample for his research is to get students and teachers at the local all-boys' school, and as a result he ends up with a sample that is 95% male and 5% female (some female teachers were included in the sample). It sounds like the method he is using to obtain the sample is .