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Sampling for qualitative research
FEBRUARY 2001, VOL 73, NO 2 RESEARCH CORNER
Sampling for qualitative research
0
ualitative sampling can be confusing, especially if one’s knowledge regarding sampling methods originates from a framework of statistical inference. Although not based on statistical theories, some basic sampling principles for conducting qualitative research do exist. This column will describe some strategies for quantitative sampling. A brief overview of quantitative sampling will provide a background with which one can contrast qualitative sampling.
QUANTITATIVE VERSUS QUALITATIVE SAMPLING Quantitative researchers strive to collect large amounts of data using random selection methods. The rationale for this premise is drawn from inferential statistics and assumes that samples are drawn from a particular population. The larger the random sample drawn from a given population, the less variation in each selected sample and the more representative of the given population.‘ Analysis performed on these data can be descriptive (eg, descriptions of the sample) or inferential (eg, an estimation of population parameters derived from the sample). Numerical data are used to describe the sample, examine relationships, and determine cause and effect relationships between variables.2 Quantitative data and findings have been criticized for being reductionistic and removed from human experience. In contrast,
qualitative methods seek to represent holism and to provide contextual knowledge of the phenomenon being studied. One goal of qualitative research is to increase understanding of a phenomenon as opposed to generalizing data extrapolated from the sample to the population at large. Rather than having a quantitative research outcome of generalized findings, qualitative researchers have an onus of richly describing the findings so they can be transferred to other situations. The qualitative researcher’s responsibility includes providing enough description about the context of the sample so that others may adequately judge whether the findings apply to their own situations.
UNDERSTANDING QUALITATIVE SAMPLING A research consumer must ask whether there is enough evidence from one qualitative study to transfer the knowledge to another group of people with a similar experience or even to another type of phenomenon. For example, assume I read a study about middle-age college students’ experiences with test anxiety. The researcher provided enough description and context of the sample and findings that, after reading it, I found it could apply to my 15-year-old daughter when she takes tests. I also concluded the findings apply and provide insight into preoperative patient anxiety. Of course, these 494 AORN JOURNAL
are potential areas for future research; however, as an individual searching for knowledge, this study provided increased understanding of the phenomenon of anxiety. Understanding what purpose research will serve should be a decisive factor in selecting a qualitative sample. A researcher has many sampling choices available that may stem from theory, method, or simple practicalities, such as time and money. A sample, therefore, is chosen purposefully, and many sampling strategies can be used.3Although many sampling strategies are discussed in the literature, only some will be described in this c01umn.~A hypothetical study will be described to illustrate some of the sampling strategies and decisions that a qualitative researcher may make. As the researcher explores and decides which actions to take, these decisions must be documented in a research journal, which serves as an audit trail for credibility defense.
AN EXAMPLE For illustration, let us suppose that a researcher is a graduate nursing student who has been introduced to phenomenology. This nurse researcher has been practicing in women’s health for 12 years and wants to learn about the lived experiences of women needing hysterectomies. Experientially, this researcher has noticed an emotional
FEBRUARY 2001, VOL 73, NO 2
difference in women with infertility who need hysterectomies versus fertile women who need hysterectomies. By narrowing the interest to women with infertility, the sample also narrows. If the researcher was interested in all women who need hysterectomies, the sample would include women from many ethnic groups, economic backgrounds, and age groups (eg, adolescents, older adults). Although the focus of this hypothetical study solves the gender issue, other studies may need to address gender inclusion criteria. Usually, the broader the phenomenon under study, the larger the sample a researcher would need. To adequately sample to understand the experiences of all patients needing hysterectomies, the researcher would need representative women from different ethnic groups, socioeconomic classes, and age groups. This undertaking could be too broad and unrealistic. It would behoove a qualitative researcher to build one study upon another during his or her research career. For example, homogenous samples that describe particular subgroups in detail could reflect the lived experiences of patients needing hysterectomies, such as African American perimenopausal women, adolescents, recent Hispanic immigrants, patients diagnosed with uterine cancer, patients with postpartum hemorrhage, and this current hypothetical study of infertility patients. After many studies have been completed, a meta-analysis could be performed on the findings from each study to determine
commonalties of the lived experience for all patients needing hysterectomies. In our hypothetical study, the researcher decides to use criterion sampling. Two selection criteria could include self-report of
Recruiting people of various backgrounds for a research study is termed maximum variation. infertility and women age 35 or older. Again, these criteria narrow the sample. For sample recruitment, suppose this researcher lives in a large urban area with diverse ethnic representation. The researcher, therefore, will have the ability to recruit from different areas, thus capturing different cultures and socioeconomic classes. This type of sampling is termed maximum v a r i a t i ~ nIn . ~addition to the previously cited sampling criteria, another common sampling criterion used is the requirement that participants must be English speaking because monolingual researchers often cannot and do not want to incur the expense of hiring translators. OTHER FACTORS TO CONSIDER Qualitative researchers also must determine how many participants are necessary. There is no single answer to this question. One indicator could be the sam497 AORN JOURNAL
ple size of similar types of published studies. As previously stated, another factor is the breadth and depth of the phenomenon under study. The sample size also may be related to restraints of the researcher’s time, budget, and geographic location. When the phenomenon under study is extremely narrow, it may be difficult to recruit enough participants. Sometimes a snowballing sampling technique is used to recruit additional participants from those already selected to participate in the study. For example, if the researcher interviews a patient with infertility who underwent a hysterectomy, the researcher may ask this patient whether she knows of anyone else with a similar experience. This patient’s recommendation may lead to the addition of another research participant. A pilot study always is recommended. Pilot data may contribute to additional sampling criteria. Consider another hypothetical example with this study. After interviewing four or five participants in a pilot study, the researcher discovers that two of the participants had recently gone through a divorce. Through data analysis, the researcher finds that most of the data and the interview transcript relate to the issue of divorce rather than infertility or hysterectomy. The researcher, therefore, may add a sampling criterion that requires participants to be married to the same partner for the previous 10 years. Qualitative researchers frequently perform data analysis concurrently with data collection. So, in the midst of a study, how can the researcher know when there are enough participants and, thus,
FEBRUARY 2001, VOL 73, NO 2
enough data? The completion of data collection and the resulting subject size may be the result of data saturation. After enough data have been collected to determine themes or categories, the researcher may decide that if the next few participants’ experiences are captured by the existing themes or categories, the phenomenon of study is saturated or complete. This means that the researcher’s construct represents NOTES 1. E W Minium, B M King, G R Bear, Statistical Reasoning in Psychology and Education, third ed (New York John Wiley & Sons, Inc, 1993) 23. 2. N Bums, S K Grove, The Practice of Nursing Research: Conduct, Critique & Utilization, sec-
the phenomenon of study, and no further data collection is necessary. CONCLUSION In conclusion, qualitative researchers perform sampling with a purpose. It is incumbent on the researcher to describe the sample in regards to gender, ethnicity, age, socioeconomic class, and any other relevant criteria so research consumers can understand how and why this sample ond ed (Philadelphia: W B Saunders Co, 1993) 451-492. 3. M Q Patton, Qualitative Evaluation and Research Methods,
second ed (Newbury Park, Calif: Sage Publications, 1990) 143-198; I T Coyne, “Sampling in qualitative research. Purposeful and theoretical sampling; merging or clear bound-
was chosen. Many strategies influence sample size and selection. The researcher must document the decision-making process involved in qualitative sampling to provide credibility for the research findings. Next month’s column will address evaluating the credibility of qualitative research. MICHELLE BYRNE RN, MS, PHD, CNOR NURSING RESEARCH COMMIITEE
aries?’ Journal of Advanced Nursing 26 (September 1997) 623-630. 4. Ibid. 5. M Sandelowski, “Sample size in qualitative research,” Research in Nursing and Health 18 (April 1995) 179-183.
National Muscular Dystrophy Registry Established The National Institute of Arthritis and Musculoskeletal and Skin Diseases and the National Institute of Neurological Disorders and Stroke have established a national registry for myotonic dystrophy (DM) and facioscapulohumeral dystrophy (FSHD), according to a Dec 11,2000, news release from the National Institutes of Health (NIH). Registry scientists will identify and classify patients with clinically diagnosed forms of DM and FSHD and store their medical and family histories. The registry will be a central information source for researchers studying these diseases. Recommendations regarding enrollment criteria, monitoring and improving ways to recruit patients and investigators, and assessing programs will be made by the registry’s scientific advisory committee. This committee also will revise and extend data collection and handling methods and determine possible clinical studies. There are nine types of muscular dystrophy, including DM and FSHD. These diseases can be detected at birth, may be passed from one generation to the next, and may cause progressive, disabling weakness. Additionally, DM may result
in sudden death. Congenital, juvenile, adult, and late onset are the four types of DM. The disease is marked by a slow progression of weakness and muscle wasting affecting the face, feet, hands, neck, and glandular system. Muscles are unable to relax after contraction. Both males and females may be affected with DM, and the cause of the disease is unknown. In contrast, FSHD may progress either slowly or rapidly. It is marked by weakness in the facial muscles and weakness and wasting of the shoulders and upper arms. Both males and females may be affected. A child whose parent is affected has a 50% risk of inheriting the disease. The cause of FSHD, the third most common genetic disease of skeletal muscle, is unknown. Currently, patient enrollment for the registry is scheduled to begin in the fall of 2001. The project is funded under the NIH contract NO1-AR-02250. National Regishy Established for Two Muscular Dystrophy Types (news release, Bethesdo, Md: National Institutes of Health) 1-2.Available from http://tvww.nih.gov/newdpr /dec2OOO/niams- 1 1.htm. Accessed 1 1 Dec 2000.
498 AORN JOURNAL
Sampling for qualitative research
0
ualitative sampling can be confusing, especially if one’s knowledge regarding sampling methods originates from a framework of statistical inference. Although not based on statistical theories, some basic sampling principles for conducting qualitative research do exist. This column will describe some strategies for quantitative sampling. A brief overview of quantitative sampling will provide a background with which one can contrast qualitative sampling.
QUANTITATIVE VERSUS QUALITATIVE SAMPLING Quantitative researchers strive to collect large amounts of data using random selection methods. The rationale for this premise is drawn from inferential statistics and assumes that samples are drawn from a particular population. The larger the random sample drawn from a given population, the less variation in each selected sample and the more representative of the given population.‘ Analysis performed on these data can be descriptive (eg, descriptions of the sample) or inferential (eg, an estimation of population parameters derived from the sample). Numerical data are used to describe the sample, examine relationships, and determine cause and effect relationships between variables.2 Quantitative data and findings have been criticized for being reductionistic and removed from human experience. In contrast,
qualitative methods seek to represent holism and to provide contextual knowledge of the phenomenon being studied. One goal of qualitative research is to increase understanding of a phenomenon as opposed to generalizing data extrapolated from the sample to the population at large. Rather than having a quantitative research outcome of generalized findings, qualitative researchers have an onus of richly describing the findings so they can be transferred to other situations. The qualitative researcher’s responsibility includes providing enough description about the context of the sample so that others may adequately judge whether the findings apply to their own situations.
UNDERSTANDING QUALITATIVE SAMPLING A research consumer must ask whether there is enough evidence from one qualitative study to transfer the knowledge to another group of people with a similar experience or even to another type of phenomenon. For example, assume I read a study about middle-age college students’ experiences with test anxiety. The researcher provided enough description and context of the sample and findings that, after reading it, I found it could apply to my 15-year-old daughter when she takes tests. I also concluded the findings apply and provide insight into preoperative patient anxiety. Of course, these 494 AORN JOURNAL
are potential areas for future research; however, as an individual searching for knowledge, this study provided increased understanding of the phenomenon of anxiety. Understanding what purpose research will serve should be a decisive factor in selecting a qualitative sample. A researcher has many sampling choices available that may stem from theory, method, or simple practicalities, such as time and money. A sample, therefore, is chosen purposefully, and many sampling strategies can be used.3Although many sampling strategies are discussed in the literature, only some will be described in this c01umn.~A hypothetical study will be described to illustrate some of the sampling strategies and decisions that a qualitative researcher may make. As the researcher explores and decides which actions to take, these decisions must be documented in a research journal, which serves as an audit trail for credibility defense.
AN EXAMPLE For illustration, let us suppose that a researcher is a graduate nursing student who has been introduced to phenomenology. This nurse researcher has been practicing in women’s health for 12 years and wants to learn about the lived experiences of women needing hysterectomies. Experientially, this researcher has noticed an emotional
FEBRUARY 2001, VOL 73, NO 2
difference in women with infertility who need hysterectomies versus fertile women who need hysterectomies. By narrowing the interest to women with infertility, the sample also narrows. If the researcher was interested in all women who need hysterectomies, the sample would include women from many ethnic groups, economic backgrounds, and age groups (eg, adolescents, older adults). Although the focus of this hypothetical study solves the gender issue, other studies may need to address gender inclusion criteria. Usually, the broader the phenomenon under study, the larger the sample a researcher would need. To adequately sample to understand the experiences of all patients needing hysterectomies, the researcher would need representative women from different ethnic groups, socioeconomic classes, and age groups. This undertaking could be too broad and unrealistic. It would behoove a qualitative researcher to build one study upon another during his or her research career. For example, homogenous samples that describe particular subgroups in detail could reflect the lived experiences of patients needing hysterectomies, such as African American perimenopausal women, adolescents, recent Hispanic immigrants, patients diagnosed with uterine cancer, patients with postpartum hemorrhage, and this current hypothetical study of infertility patients. After many studies have been completed, a meta-analysis could be performed on the findings from each study to determine
commonalties of the lived experience for all patients needing hysterectomies. In our hypothetical study, the researcher decides to use criterion sampling. Two selection criteria could include self-report of
Recruiting people of various backgrounds for a research study is termed maximum variation. infertility and women age 35 or older. Again, these criteria narrow the sample. For sample recruitment, suppose this researcher lives in a large urban area with diverse ethnic representation. The researcher, therefore, will have the ability to recruit from different areas, thus capturing different cultures and socioeconomic classes. This type of sampling is termed maximum v a r i a t i ~ nIn . ~addition to the previously cited sampling criteria, another common sampling criterion used is the requirement that participants must be English speaking because monolingual researchers often cannot and do not want to incur the expense of hiring translators. OTHER FACTORS TO CONSIDER Qualitative researchers also must determine how many participants are necessary. There is no single answer to this question. One indicator could be the sam497 AORN JOURNAL
ple size of similar types of published studies. As previously stated, another factor is the breadth and depth of the phenomenon under study. The sample size also may be related to restraints of the researcher’s time, budget, and geographic location. When the phenomenon under study is extremely narrow, it may be difficult to recruit enough participants. Sometimes a snowballing sampling technique is used to recruit additional participants from those already selected to participate in the study. For example, if the researcher interviews a patient with infertility who underwent a hysterectomy, the researcher may ask this patient whether she knows of anyone else with a similar experience. This patient’s recommendation may lead to the addition of another research participant. A pilot study always is recommended. Pilot data may contribute to additional sampling criteria. Consider another hypothetical example with this study. After interviewing four or five participants in a pilot study, the researcher discovers that two of the participants had recently gone through a divorce. Through data analysis, the researcher finds that most of the data and the interview transcript relate to the issue of divorce rather than infertility or hysterectomy. The researcher, therefore, may add a sampling criterion that requires participants to be married to the same partner for the previous 10 years. Qualitative researchers frequently perform data analysis concurrently with data collection. So, in the midst of a study, how can the researcher know when there are enough participants and, thus,
FEBRUARY 2001, VOL 73, NO 2
enough data? The completion of data collection and the resulting subject size may be the result of data saturation. After enough data have been collected to determine themes or categories, the researcher may decide that if the next few participants’ experiences are captured by the existing themes or categories, the phenomenon of study is saturated or complete. This means that the researcher’s construct represents NOTES 1. E W Minium, B M King, G R Bear, Statistical Reasoning in Psychology and Education, third ed (New York John Wiley & Sons, Inc, 1993) 23. 2. N Bums, S K Grove, The Practice of Nursing Research: Conduct, Critique & Utilization, sec-
the phenomenon of study, and no further data collection is necessary. CONCLUSION In conclusion, qualitative researchers perform sampling with a purpose. It is incumbent on the researcher to describe the sample in regards to gender, ethnicity, age, socioeconomic class, and any other relevant criteria so research consumers can understand how and why this sample ond ed (Philadelphia: W B Saunders Co, 1993) 451-492. 3. M Q Patton, Qualitative Evaluation and Research Methods,
second ed (Newbury Park, Calif: Sage Publications, 1990) 143-198; I T Coyne, “Sampling in qualitative research. Purposeful and theoretical sampling; merging or clear bound-
was chosen. Many strategies influence sample size and selection. The researcher must document the decision-making process involved in qualitative sampling to provide credibility for the research findings. Next month’s column will address evaluating the credibility of qualitative research. MICHELLE BYRNE RN, MS, PHD, CNOR NURSING RESEARCH COMMIITEE
aries?’ Journal of Advanced Nursing 26 (September 1997) 623-630. 4. Ibid. 5. M Sandelowski, “Sample size in qualitative research,” Research in Nursing and Health 18 (April 1995) 179-183.
National Muscular Dystrophy Registry Established The National Institute of Arthritis and Musculoskeletal and Skin Diseases and the National Institute of Neurological Disorders and Stroke have established a national registry for myotonic dystrophy (DM) and facioscapulohumeral dystrophy (FSHD), according to a Dec 11,2000, news release from the National Institutes of Health (NIH). Registry scientists will identify and classify patients with clinically diagnosed forms of DM and FSHD and store their medical and family histories. The registry will be a central information source for researchers studying these diseases. Recommendations regarding enrollment criteria, monitoring and improving ways to recruit patients and investigators, and assessing programs will be made by the registry’s scientific advisory committee. This committee also will revise and extend data collection and handling methods and determine possible clinical studies. There are nine types of muscular dystrophy, including DM and FSHD. These diseases can be detected at birth, may be passed from one generation to the next, and may cause progressive, disabling weakness. Additionally, DM may result
in sudden death. Congenital, juvenile, adult, and late onset are the four types of DM. The disease is marked by a slow progression of weakness and muscle wasting affecting the face, feet, hands, neck, and glandular system. Muscles are unable to relax after contraction. Both males and females may be affected with DM, and the cause of the disease is unknown. In contrast, FSHD may progress either slowly or rapidly. It is marked by weakness in the facial muscles and weakness and wasting of the shoulders and upper arms. Both males and females may be affected. A child whose parent is affected has a 50% risk of inheriting the disease. The cause of FSHD, the third most common genetic disease of skeletal muscle, is unknown. Currently, patient enrollment for the registry is scheduled to begin in the fall of 2001. The project is funded under the NIH contract NO1-AR-02250. National Regishy Established for Two Muscular Dystrophy Types (news release, Bethesdo, Md: National Institutes of Health) 1-2.Available from http://tvww.nih.gov/newdpr /dec2OOO/niams- 1 1.htm. Accessed 1 1 Dec 2000.
498 AORN JOURNAL