Jon surveyed his school to find their preference on a school mascot. He asked everybody on the football team. Identify the population in this situation.

Because everyone he surveyed was on the football team, the sample population all have that in common, and so this sample is not a random one. I don't remember the name of this particular type of sample. You'd benefit from summarizing your textbook explanations of different types of samples and making sure that you can easily give an example of each and every one of them.

Controlled population