ABSTRACT
A population is a defined collection of all subjects about which data will be collected and conclusions drawn. For example, the US census attempts to obtain information from all residents. A sample is a subset of a specified population. Often, researchers obtain data from a sample rather than from the entire population. When each member of the population is given an equal chance of being included in the sample, the sample is called a probability sample. Statistical theory can be used to help infer characteristics of the population from a probability sample. Researchers purposively seek out subjects that they believe are representative or, at least, important sources of information. Results based on nonprobability samples should be viewed with caution for example, there may have been major demographic changes in counties that previously voted for the winners since the last election.
