Researchers must understand the limitations and assumption contained in reputable work already done in the field. While a Population is the entire group for which results may be considered, the Sample must be a representative subset of that entire group. The purpose of the sample is to provide information that is generalizable to the larger group. A good sample must reflect the population from which it is drawn accurately. Reliable samples represent populations accurately.
Valid selections generalize to the larger populations.
Error Systematic, or researcher biased, is introduced into a sample due to some fault of the researcher. Reliability measures and coefficients indicate the degree of consistency with which the instrument measures what it claims to. Validity measures indicate WHAT a test measures, and the degree to which it may be presumed that it will measure what it claims to. Validity is assessed in connection with Content area, hypothetical Construct, and Concurrence against previously validated instruments. Inferential analyses are used to help the researcher make inferences about the population from which data were derived.