Click here to purchase the entire book in PDF format. Sample A sample in statistics is a subset of the population. If we looked at the ages of five of the thirty students in the class mentioned above, we'd be using a sample of the population. It's comprised of n measurements, each of which is labeled $x_{i}$ (the i refers to the index number of the measurement) Samples are often used in statistics because you can't know everything about everybody. For example, if we want to know something about the ages of all grade 10 students in Canada, we could find out the actual age of every grade student in Canada (the population) or we could just look at the grade 10 classes in a couple of schools (a sample) and hope that the rest of the population has similar characteristics.