Sample issues, methodological implications, and best practices.

This paper discusses issues and best practices associated with participant samples. First, a review of the fundamental differences between probability and nonprobability samples is explored. Given the ubiquitous presence of nonprobability samples in most psychological research, best practices are suggested to deal with the violations of assumptions inherent in them. Next, the perennial concern of how large a sample size should be is considered. After the data have been collected, the matters of handling response rates, missing data, outliers, and demographically based subgroups frequently need to be addressed prior to data analysis. Recommendations are presented for each issue that are consistent with expectations of editors, reviewers, and readers of the research.