2) Population Specification Error (Systematic Sample Design Error)

Population specification error occurs when a researcher fails to clearly define the population of interest. For example, when the researcher does not specify which age group or geographical area should be included in the study, resulting in the sample being biased or incomplete. This type of error is particularly common with systematic sampling designs, where a sample of individuals is selected from a fixed list of units, rather than from the entire population of interest. Systematic sampling designs can result in population specification errors when the selection criteria do not capture the full range of units within the population of interest.