Objectives To identify sampling methods
To recognize bias in samples and surveys
A population is all the members of a set. A sample is part of a population. If you determine a sample carefully, it can give a good estimate of the total population.
Essential Understanding You can get good statistical information about a population by studying a sample of the population.
Suppose you want to know what percent of all voters in your city favor a tax increase to pay for school improvements. It likely would be impossible to ask an opinion of every voter. So instead you select a sample of the voters to estimate the percentage who favor the idea.
You can define different sample types by the methods used to select them.
For a convenience sample, select any members of the population who are conveniently and readily available.
For a self-selected sample, select only members of the population who volunteer for the sample.
For a systematic sample, order the population in some way, and then select from it at regular intervals.
In a random sample, all members of the population are equally likely to be chosen.