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Sampling in Qualitative Research

Sampling is the deliberate selection of the most appropriate participants to be included in the study, according to the way that the theoretical needs of the study may be met by the characteristics of the participants. In qualitative inquiry, attention to sampling is crucial for the attainment of rigor and takes place throughout the research process.

Types of Samples

Samples are classified according to the means by which participants are selected from the population. In qualitative inquiry, this selection procedure is a deliberate rather than a random process.

Convenience Sample

Convenience sampling is selecting the sample by including participants who are readily available and who meet the study criteria. A convenience sample may be used at the beginning of the sampling process, when the investigator does not know the pertinent characteristics for criteria for sample selection, or it is used when the number of participants available is small. For example, we may be exploring the experience of children who have had cardiac surgery, or of ventilator-dependent schoolchildren, and this patient population within a given geographic area is low. When all available participants have been included in the study (e.g., all children in a particular classroom), it is referred to as a total sample.

Minimally, to be included as a participant in a qualitative interview, participants must be willing and able to reflect on an experience, articulate their experience, and have time available without interruption to be interviewed. Problems may arise when using a total sample: When a participant is invited or volunteers to be interviewed, the researcher does not know if the participant will meet that minimal criterion for a “good” participant. If we include all participant interviews in the initial sample, we have not used any screening procedures. On the other hand, if the researcher starts an interview and finds that the participant is not a good informant and does not have the anticipated experiences nor knows the information we are seeking, then we practice secondary selection (Morse, 1989). In this case, the researcher politely completes the interview. The tape is labeled and set aside. The researcher does not waste time and research funds transcribing the tape. The interview is not incorporated into the data set, but neither is the tape erased. The audiotape is stored, just in case it makes sense and is needed at a later date.

Theoretical Sample

This is the selection of participants according to the theoretical needs of the study. If the researcher has identified a comparative research question and knows the dimensions of the comparison to be conducted, theoretical sampling may be used at the commencement of data collection. For example, the researcher may use a two-group design comparing participants—for instance, comparing single mothers who are continuously receiving unemployment with those who periodically work and sporadically claim benefits. Thus, theoretical sampling enables comparison of the two sets of data and the identification of differences between the two groups. As analysis proceeds, categories and themes form, some areas of analysis will be slower to saturate than others or may appear thin. A theoretical sample is the deliberate seeking of participants who have particular knowledge or characteristics that will contribute data to categories with inadequate data, referred to as thin areas of analysis. Alternatively, the investigator may develop a conjecture or hypothesis that could be tested by seeking out and interviewing people with characteristics that may be compared or contrasted with other participants. An example may be the findings from the previous study of single, unemployed mothers, which revealed that those mothers who returned to work were more resilient than those who remained unemployed. Because this sample had too few participants to verify this emerging finding, the researchers must then deliberately seek out participants who fit that criterion, in order to saturate those categories. This type of theoretical sampling, in which participants are deliberately sought according to individual characteristics, is sometimes also referred to as purposive sampling.

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