Sampling

Steps in the Sampling Process

The sampling process can be diagrammed as shown in Figure 1.

Identifying the population or entire group of interest is an important first step in designing the sampling method. This entire population is often referred to as the theoretical or target population because it includes all of the participants of theoretical interest to the researcher. These are the individuals about which the researcher is interested in making generalizations. Examples of possible theoretical populations are all high school principals in the United States, all couples over age 80 in the world, and all adults with chronic fatigue syndrome. It is hardly ever possible to study the entire theoretical population, so a portion of this theoretical population that is accessible (the accessible population or sampling frame) is identified. Researchers define the accessible population/sampling frame based on the participants to which they have access. Examples of accessible populations might be the high school principals in the state of Colorado, couples over age 80 who participate in a community activity targeting seniors, or patients who have visited a particular clinic for the treatment for chronic fatigue syndrome. From this accessible population, the researcher might employ a sampling design to create the selected sample, which is the smaller group of individuals selected from the accessible population. These individuals are asked by the researcher to participate in the study. For example, one might sample high school principals by selecting a random sample of 10 school districts within the state of Colorado. In other cases, the accessible population might be small enough that the researcher selects all these individuals as the selected “sample.” For example, a researcher studying couples older than age 80 who participate in a particular community activity could choose to study all the couples in that group rather than only some of them. In this case, the accessible population and the selected sample are the same. A third example of an accessible population could be if the patients treated during a certain 3-month time period were chosen as the selected sample from the accessible population of all individuals seeking treatment for chronic fatigue syndrome at a particular clinic.

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