Fundamentals of Statistics 1: Basic Concepts :: A Population and Sample
If you talk to a statistician for about 15 seconds, you'll hear the term population and sample used quite a bit. A bit of explanation is in order.

If we wanted to know if a new way of teaching Algebra to 9th graders improved their test grades, we could give half of all ninth graders the new method and half the old. However, it would take quite a while and cost a lot of money to teach and test ALL ninth graders.

Instead, we could take a sample of 9th graders, give half of the sample the new method and half the old and see if the method improves scores on Algebra tests. The scores we get from our sample of 9th graders we'd use to make our best guess about the whole untested population of 8th graders.  The population is all 9th graders and our sample is just the 9th graders we select for our experiment.

Populations don't have to be people or static things. In the example with the 9th grade algebra, we really have 2 populations, 1 that receives the new method and another population that doesn't. If we administered say a third type of training to another sample of 9th graders, we'd then have a third population we'd need to make inferences about.

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