Sampling
Definitiion
Sampling is the process of systematically selecting that which will be examined during the course of a study.
Types of Sampling Approaches
There are a wide range of sampling approaches. We use Miles and Huberman (1994), Patton (2001), Kuzel (1999) and Glaser and Strauss (1967) to provide brief descriptions of different sampling strategies.
Please keep in mind that a strong research design and analytical approach will:
- Incorporate more than one of the sampling strategies described below
- Include an iterative sampling approach whereby the research team moves back and forth (iterating) between sampling and analyzing data such that preliminary analytical findings shape subsequent sampling choices.
With the exception of random and convenience sampling, all of the sampling strategies defined below are considered purposeful sampling strategies - where researchers select cases with a particular purpose or goal in mind.
For an interesting discussion of the distinction between purposeful and theoretical sampling and the use of these terms see Coyne (1997).
For other sampling typologies please see Morse (1991) and Sandelowski (1992; 1995).
How big should a sample be?
Sample size is an important consideration in qualitative research. Typically, researchers want to continue sampling until having achieved informational redundancy or saturation -- the point at which no new information or themes are emerging from the data.
To know if informational redundancy or saturation is reached implies and is founded on the assumption that data collection and analysis are going hand-in-hand. In other words, data is collected and analyzed, at least in a preliminary fashion, and this analysis informs subsequent data collection decisions.
It is important to keep in mind that saturation or informational redudancy can be reached prematurely if:
- one's sampling frame is too narrow
- one's analytical perspective is skewed or limited
- the method employed is not resulting in rich, in depth information
- the researcher is unable to get beyond the surface or 'status quo' with respondents
As Sandelowski (1995) points out, "determining adequate sample size in qualitative research is ultimately a matter of judgement and experience" and researchers need to evaluate the quality of the information collected in light of the uses to which it will be put, and the research method, sampling and analytical strategy employed.
Flexibile research designs that build in iterative sampling and analysis strategies, encourage reflexivity and collaboration may yield better results.
References
Altmann, J. (1974). "Observational study of behaviour: sampling methods." Behavior, 49. pp. 227-267.
Coyne, IT. (1997). "Sampling in qualitative research: Purposeful and theoretical sampling; merging or clear boundaries." Journal of Advanced Nursing 26, 623-630.
Glaser, B. & Strauss, A. (1967). The Discovery of Grounded Theory: Strategies for Qualitative Research. Chicago: Aldine.
Kuzel, AJ. (1999). "Sampling in qualitative inquiry." In BF Crabtrree and WL Miller (Eds.) Doing Qualitative Research (second edition). Thousand Oaks, CA: Sage Publlications (pp. 33-45).
Lincoln, YS. & Guba, EG. (1985). Naturalistic Inquiry. Newbury Park, CA: Sage Publications.
Luborski, MR. & Rubinstein, RL. (1995). "Sampling in qualitative research: Rationale, issues and methods." Research on Aging 17(1), 89-113.
Malterud, K. (2001). "Qualitative research: Standards, challenges and guidelines." The Lancet. 358: pp. 483-488.
Miles, MB. & Huberman, AM. (1994). Qualitative Data Analysis (2nd edition). Thousand Oaks, CA: Sage Publications.
Morse, JM. (1991). "Strategies for sampling." In Qualitative Nursing Research: A Contemporary Dialogue (JM Morse, Ed.) Newbury Park, CA: Sage Publications, pp. 127-145.
Patton, MQ. (1999). "Enhancing the quality and credibility of qualitative analysis." HSR: Health Services Research. 34 (5) Part II. pp. 1189-1208.
Patton, MQ. (2001). Qualitative Research and Evaluation Methods (2nd Edition). Thousand oaks, CA: Sage Publications.
Popay, J., Rogers, A., & Williams, G. (1998). "Rationale and standards for the systematic review of qualitative literature in health services research." Qualitative Health Research. 8(3), 341-351.
Sandelowski, M. (1995). "Sample size in qualitative research." Research in Nursing and Health. 18, 179-183.
Sandelowski, M., Holditch-Davis, D. & Harris, BG. (1992). "Using qualitative and quantitative methods: the transition to parentood of infertile couples." In Qualitative Methods in Family Research (JF. Gilgun, K. Daly & G Handel Eds.) Newbury Park, CA: Sage Publication, pp. 301-323.
Strauss, A. & Corbin, J. (1994). "Grounded Theory Methodology." In NK Denzin & YS Lincoln (Eds.) Handbook of Qualitative Research (pp. 217-285). Thousand Oaks, Sage Publications.
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