Qualitative Sampling Methods
The following module describes common methods for collecting qualitative data.
- Describe common types of qualitative sampling methodology.
- Explain the methods typically used in qualitative data collection.
- Describe how sample size is determined.
The three main types of data collected and analyzed in qualitative research include in-depth interviews, direct observation, and written documents. These are discussed in greater detail in the Qualitative Ready module covering data types. In order to collect these types of data for a study, a target population, community, or study area must be identified first. It is not possible for researchers to collect data from everyone in a sample area or community. Therefore, the researcher must gather data from a sample, or subset, of the population in the study. In quantitative research, the goal would be to conduct a random sampling that ensured the sample group would be representative of the entire population, and therefore, the results could be generalized to the entire population. The goal of qualitative research is to provide in-depth understanding and therefore, targets a specific group, type of individual, event or process. To accomplish this goal, qualitative research focus on criterion-based sampling techniques to reach their target group. There are three main types of qualitative sampling: purposeful sampling, quota sampling, and snowballing sampling. The following descriptions describe the reasons for choosing a particular method.
- Purposeful Sampling is the most common sampling strategy. In this type of sampling, participants are selected or sought after based on pre-selected criteria based on the research question. For example, the study may be attempting to collect data from lymphoma patients in a particular city or county. The sample size may be predetermined or based on theoretical saturation, which is the point at which the newly collected no longer provides additional insights. Click on the following link for a desciption of types of purposeful sampling: Types of Purposeful Sampling.
- Quota Sampling is a sampling technique whereby participant quotas are preset prior to sampling. Typically, the researcher is attempting to gather data from a certain number of participants that meet certain characteristics that may include things such as age, sex, class, marital status, HIV status, etc. Click here for more information on this type of sampling: Quota Sampling.
- Snowball Sampling is also known as chain referral sampling. In this method, the participants refer the researcher to others who may be able to potentially contribute or participate in the study. This method often helps researchers find and recruit participants that may otherwise be hard to reach. For more information, click here: Snowball Sampling.
A note on sample size - Once a sampling method has been determined, the researcher must consider the sample size. In qualitative studies, sampling typically continues until information redundancy or saturation occurs. This is the point at which no new information is emerging in the data. Therefore, in qualitative studies is it critical that data collection and analysis are occurring simultaneously so that the researcher will know when the saturation point is reached. It is important to understand that the saturation point may occur prematurely if the researcher has a narrow sampling frame, a skewed analysis of the data, or poor methodology. Because of this, the researcher must carefully create the research question, select an appropriate target group, eliminate his or her own biases and analyze data continuously and thoroughly throughout the process to bring validity to the data collected.
The following slideshare presentation, Collecting Qualitative Data, and the Resource Links on this page provide additional insight into qualitative sampling.
- Berg, B. L., & Lune, H. (2004). Qualitative research methods for the social sciences (Vol. 5). Boston: Pearson.
- Coyne, I. T. (1997). Sampling in qualitative research. Purposeful and theoretical sampling; merging or clear boundaries?. Journal of advanced nursing, 26(3), 623-630.
- Malterud, K. (2001). Qualitative research: standards, challenges, and guidelines. The lancet, 358(9280), 483-488.
- Marshall, M. N. (1996). Sampling for qualitative research. Family practice, 13(6), 522-526.
- Patton, M. Q. (2005). Qualitative research. John Wiley & Sons, Ltd.
- Sandelowski, Margarete. "Sample size in qualitative research." Research in nursing & health2 (1995): 179-183.
- Strauss, A., & Corbin, J. M. (1990). Basics of qualitative research: Grounded theory procedures and techniques. Sage Publications, Inc.