1.12 Potential Problems with Sampling

Identify potential sources of bias in sampling methods.

• Bias in a sampling method is a systematic error in the sampling procedure that results in a statistic being consistently larger or consistently smaller than the parameter the statistic is used to estimate.
• Voluntary response bias is a bias that may occur when a sample consists entirely of volunteers.
• Undercoverage bias may occur when the sampling method fails to include part of the population or a part of the population is less likely to be selected based on the sampling method.
• Nonresponse bias may occur because of a failure to obtain responses from some individuals chosen to be sampled. The respondents and nonrespondents could differ significantly in ways that are important for the study.
• Response bias may occur when responses to a survey or measurements of observational units tend to differ from the “true” value in one direction. Examples include questions that are confusing or leading (question wording bias) or self-reported responses.
• Nonrandom sampling methods (e.g., samples chosen by convenience or voluntary response) introduce potential bias because they do not use random chance to select the individuals.