Judgment Sampling

Judgment sampling (a type of purposive sampling) occurs when units are selected for inclusion in a study based on the professional judgment of the researcher. This is in contrast to probability sampling techniques in which units are drawn with some probability (e.g., randomly) from the population of interest. This entry describes the common forms of judgment sampling and discusses their advantages and limitations.

Judgment sampling may be used for a variety of reasons. In general, the goal of judgment sampling is to deliberately select units (e.g., individual people, events, objects) that are best suited to enable researchers to address their research questions. This is often done when the population of interest is very small, or desired characteristics of units are very rare, making probabilistic sampling infeasible. Judgment sampling is often associated with qualitative and mixed-method study designs.

In some cases, it may be possible to sample the entire population of interest; this is referred to as “total population sampling.” For example, college professors may be interested in the perspectives of [Page 914]their own graduate students or researchers may be interested in the perspectives of current four-star Army generals (of which there are only 12).

Another common case, also referred to as “maximum variation sampling,” involves deliberately sampling subjects so as to maximize the range of one or more attributes of interest. This may be especially valuable when resources permit collecting data from only a small number of subjects, and random sampling would likely fail to capture the desired range of variation. For example, a researcher developing a survey of political attitudes may wish to conduct in-depth interviews with a small number of people to ensure that the survey is capable of capturing diverse viewpoints and may deliberately seek out people with extreme positions to do so.

A close variant of the maximum variation sampling approach, also called “extreme case sampling,” involves deliberately seeking out unusual or deviant cases (which might be missed in a simple random sample); for example, a researcher may wish to study the practices of the most highly competent or creative members of a profession (by whatever criteria). Conversely, a researcher may wish to focus only on typical cases or on units that share some set of characteristics, in which case extreme cases may be deliberately avoided.