Sample size determination in Qualitative and Quantitative Research
This section describes the conceptual basis for what the researcher will investigate, including the research questions, hypotheses, and basic research design. The introduction develops the significance of the study by describing how the study is new or different from other studies, how it addresses something that is not already known or has not been studied before, or how it extends prior research on the topic in some way. This section should also briefly describe the basic nature of the study and provide an overview of the contents of Chapter 1.
Keep in mind that you will write Chapters 1 through 3 as your dissertation proposal. However, there are changes that typically need to be made in these chapters to enrich the content or to improve the readability as you write the final dissertation manuscript. Often, after data analysis is complete, the first three chapters will need revisions to reflect a more in-depth understanding of the topic and to ensure consistency.
To ensure the quality of both your proposal and your final dissertation and reduce the time your writing needs to reflect doctoral level, scholarly writing standards from your very first draft. Each section within the proposal or dissertation should be well organized and easy for the reader to follow. Each paragraph should be short, clear, and focused. A paragraph should (1) be three to eight sentences in length, (2) focus on one point, topic, or argument, (3) include a topic sentence the defines the focus for the paragraph, and (4) include a transition sentence to the next paragraph. Include one space after each period. There should be no grammatical, punctuation, sentence structure, or APA formatting errors. Verb tense is an important consideration for Chapters 1 through 3. For the proposal, the researcher uses future tense (e.g. “The purpose of this study is to…”), whereas in the dissertation, the chapters are revised to reflect past tense (e.g. “The purpose of this study was to…”). Taking the time to ensure high quality, scholarly writing for each draft will save you time in all the steps of the development and review phases of the dissertation process so make sure to do it right the first time!
As a doctoral researcher, it is your responsibility to ensure the clarity, quality, and correctness of your writing and APA formatting. Your chair and your committee members are not obligated to edit your documents. If you do not have outstanding writing skills, you may need to identify a writing coach, editor and/or other resource to help you with writing and editing. Poorly written proposals and dissertations will be immediately suspended in the various levels of review if submitted with grammatical, structural, and/or form and formatting errors.
The quality of a dissertation is not only evaluated on the quality of writing. It is also evaluated based on the criteria that have been established for each section of the dissertation. The criteria describe what must be addressed in each section within each chapter. As you develop a section, first read the section description. Then review each criterion contained in the table below the description. Use both the overall description and criteria as you write each section. It is important that each listed criterion is addressed in a way that it is clear to your chair and committee members. You should be able to point out where each criterion is met in each section.
Prior to submitting a draft of your proposal or dissertation or a single chapter to your chair or committee members, please assess yourself on the degree to which each criterion has been met.
You need to continuously and objectively self-evaluate the quality of your writing and content for each section within the proposal or dissertation. When you have completed a comprehensive self-evaluation of your work, then you may submit your document to your chair for review. Your chair will also review each section of the proposal and dissertation and will determine when it is ready for full committee review. Keep in mind the committee review process will likely require several editorial/revisions rounds, so plan for multiple revision cycles as you develop your dissertation completion plan and project timeline. You will notice in the tables that certain columns have an X in the scoring box. As mentioned above, your chair will score all five chapters, the abstract and the reference list. Your chair and committee members will assess each criterion in their required chapters when they return the document with feedback.
Once the document has been fully approved by your chair and committee, and is approved for, your chair will approve each chapter in the courseroom.What is Sampling and Sample Size
Sampling, as it relates to research, refers to the selection of individuals, units, and/or settings to be studied. Whereas quantitative studies strive for random sampling, qualitative studies often use purposeful or criterion-based sampling, that is, a sample that has the characteristics relevant to the research question(s). For example, if you are interested in studying adult survivors of childhood sexual abuse, interviewing a random sample of 10 people may yield only one adult survivor, thus, you will essentially have a sample size of one and need to continue to randomly sample people until you have interviewed an appropriate number of who have survived childhood sexual abuse. This is not a wise use of your time.
The difference in sampling strategies between quantitative and qualitative studies is due to the different goals of each research approach. Recall that typical quantitative research seeks to infer from a sample to a population (for example, a relationship or a treatment effect). In general, you want to include a variety of types of people in a quantitative study so that it generalizes beyond those in your study. Thus, the goal of quantitative approaches can be stated as, ”empirical generalization to many.”
Qualitative research, on the other hand, typically starts with a specific group, type of individual, event, or process. As in the qualitative study of adult survivors of childhood sexual abuse example above, you would choose your sample very purposefully and include in your study only those with this particular experience. The goal of qualitative research can be stated as “in-depth understanding.”
Common Qualitative Sampling Strategies
Extreme or Deviant Case Sampling—Looks at highly unusual manifestations of the phenomenon of interest, such as outstanding success/notable failures, top of the class/dropouts, exotic events, crises. This strategy tries to select particular cases that would glean the most information, given the research question. One example of an extreme/deviant case related to battered women would be battered women who kill their abusers.
Intensity Sampling—Chooses information-rich cases that manifest the phenomenon intensely, but not extremely, such as good students/poor students, above average/below average. This strategy is very similar to extreme/deviant case sampling as it uses the same logic. The difference is that the cases selected are not as extreme. This type of sampling requires that you have prior information on the variation of the phenomena under study so that you can choose intense, although not extreme, examples. For example, heuristic research uses the intense, personal experience(s) of the researcher. If one were studying jealousy, you would need to have had an intense experience with this particular emotion; a mild or pathologically extreme experience would not likely elucidate the phenomena in the same way as an intense experience.
Maximum Variation Sampling—Selects a wide range of variation on dimensions of interest. The purpose is to discover/uncover central themes, core elements, and/or shared dimensions that cut across a diverse sample while at the same time offering the opportunity to document unique or diverse variations. For example, to implement this strategy, you might create a matrix (of communities, people, etc.) where each item on the matrix is as different (on relevant dimensions) as possible from all other items.
Homogeneous Sampling—Brings together people of similar backgrounds and experiences. It reduces variation, simplifies analysis, and facilitates group interviewing. This strategy is used most often when conducting focus groups. For example, if you are studying participation in a parenting program, you might sample all single-parent, female head of households.
Typical Case Sampling—Focuses on what is typical, normal, and/or average. This strategy may be adopted when one needs to present a qualitative profile of one or more typical cases. When using this strategy you must have a broad consensus about what is “average.” For example, if you were working to begin development projects in Third World countries, you might conduct a typical case sampling of “average” villages. Such a study would uncover critical issues to be addressed for most villages by looking at the ones you sampled.
Critical Case Sampling—Looks at cases that will produce critical information. In order to use this method, you must know what constitutes a critical case. This method permits logical generalization and maximum application of information to other cases because if it’s true of this one case, it’s likely to be true of all other case. For example, if you want to know if people understand a particular set of federal regulations, you may present the regulations to a group of highly educated people (“If they can’t understand them, then most people probably cannot”) and/or you might present them to a group of under-educated people (“If they can understand them, then most people probably can”).
Snowball or Chain Sampling—Identifies cases of interest from people who know people who know what cases are information-rich, that is, who would be a good interview participant. Thus, this is an approach used for locating information-rich cases___?” For example, you would ask for nominations, until the nominations snowball, getting bigger and bigger. Eventually, there should be a few key names that are mentioned repeatedly.
Criterion Sampling—Selects all cases that meet some criterion. This strategy is typically applied when considering quality assurance issues. In essence, you choose cases that are information-rich and that might reveal a major system weakness that could be improved.
Theory-Based or Operational Construct or Theoretical Sampling—dentifies manifestations of a theoretical construct of interest so as to elaborate and examine the construct. This strategy is similar to criterion sampling, except it is more conceptually focused. This strategy is used in grounded theory studies. You would sample people/incidents, etc., based on whether or not they manifest/represent an important theoretical or operational construct. For example, if you were interested in studying the theory of “resiliency” in adults who were physically abused as children, you would sample people who meet theory-driven criteria for “resiliency.”
Confirming and Disconfirming Sampling—Seeks cases that are both “expected” and the “exception” to what is expected. In this way, this strategy deepens initial analysis, seeks exceptions, and tests variation. In this strategy you find both confirming cases (those that add depth, richness, credibility) as well as disconfirming cases (example that do not fit and are the source of rival interpretations). This strategy is typically adopted after initial fieldwork has established what a confirming case would be. For example, if you are studying certain negative academic outcomes related to environmental factors, like low SES, low parental involvement, high teacher to student ratios, lack of funding for a school, etc. you would look for both confirming cases (cases that evidence the negative impact of these factors on academic performance) and disconfirming cases (cases where there is no apparent negative association between these factors and academic performance).
Stratified Purposeful Sampling—Focuses on characteristics of particular subgroups of interest; facilitates comparisons. This strategy is similar to stratified random sampling (samples are taken within samples), except the sample size is typically much smaller. In stratified sampling you “stratify” a sample based on a characteristic. Thus, if you are studying academic performance, you would sample a group of below average performers, average performers, and above average performers. The main goal of this strategy is to capture major variations (although common themes may emerge).
Opportunistic or Emergent Sampling—Follows new leads during fieldwork, takes advantage of the unexpected, and is flexible. This strategy takes advantage of whatever unfolds as it is unfolding, and may be used after fieldwork has begun and as a researcher becomes open to sampling a group or person they may not have initially planned to interview. For example, you might be studying 6th grade students’ awareness of a topic and realize you will gain additional understanding by including 5th grade students’ as well.
Purposeful Random Sampling—Looks at a random sample. This strategy adds credibility to a sample when the potential purposeful sample is larger than one can handle. While this is a type of random sampling, it uses small sample sizes, thus the goal is credibility, not representativeness or the ability to generalize. For example, if you want to study clients at a drug rehabilitation program, you may randomly select 10 of 300 current cases to follow. This reduces judgment within a purposeful category, because the cases are picked randomly and without regard to the program outcome.
Sampling Politically Important Cases—Seeks cases that will increase the usefulness and relevance of information gained based on the politics of the moment. This strategy attracts attention to the study (or avoids attracting undesired attention by purposefully eliminating from the sample politically sensitive cases). This strategy is a variation on critical case sampling. For example, when studying voter behavior, one might choose the 2000 election, not only because it would provide insight, but also because it would likely attract attention.
Convenience Sampling—Selects cases based on ease of accessibility. This strategy saves time, money, and effort, however, has the weakest rationale along with the lowest credibility. This strategy may yield information-poor cases because cases are picked simply because they are easy to access, rather than on a specific strategy/rationale. Sampling your co-workers, family members or neighbors simply because they are “there” is an example of convenience sampling.
Combination or Mixed Purposeful Sampling—Combines two or more strategies listed above. Basically, using more than one strategy above is considered combination or mixed purposeful sampling. This type of sampling meets multiple interests and needs. For example, you might use chain sampling in order to identify extreme or deviant cases. That is, you might ask people to identify cases that would be considered extreme/deviant and do this until you have consensus on a set of cases that you would sample.
Sample Sizes: Considerations
When determining sample size for qualitative studies, it is important to remember that there are no hard and fast rules. There are, however, at least four considerations:
What sample size will reach saturation or redundancy? That is, how large does the sample need to be to allow for the identification of consistent patterns? Some researchers say the size of the sample should be large enough to leave you with “nothing left to learn.” In other words, you might conduct interviews, and after the tenth one, realize that there are no new concepts emerging. That is, the concepts, themes, etc. begin to be redundant.
How large a sample is needed to represent the variation within target population? That is, how large must a sample be to in order to assess an appropriate amount of diversity or variation that is represented in the population of interest?
You may estimate sample size, based on the approach of the study or the data collection method used. For each category there are some related rules of thumb, represented in the tables below.
Rules of Thumb Based on Approach:
Research Approach Rule of Thumb Biography/Case Study Select one case or one person. Phenomenology Assess 10 people. If you reach saturation prior to assessing ten people you may use fewer. Grounded theory/ethnography/action research Assess 20-30 people, which typically is enough to reach saturation.
Rules of Thumb Based on Data Collection Method:
Data Collection Method Rule of Thumb Interviewing key informants Interview approximately five people. In-depth interviews
Interview approximately 30 people. Focus groups
Create groups that average 5-10 people each. In addition, consider the number of focus groups you need based on “groupings” represented in the research question. That is, when studying males and females of three different age groupings, plan for six focus groups, giving you one for each gender and three age groups for each gender. Ethnographic surveys
Select a large and representative sample (purposeful or random based on purpose) with numbers similar to those in a quantitative study.
However, after choosing one or two case study in case study approach you might want to know the exact sample size from the given population. In that case there few other techniques to determine your sample size. The image below gives us a mirror of getting sample size depending on the number of margin you are creating.
Calculating Sample size
For example: If for instance am researching on the impact of economic recession on low income earners in Abakaliki metropolises in Ebonyi state. Let say my population of study is 52% of Abakaliki population (141437). Statistics revealed that 52% of Abakaliki resident live below US $1.50 poverty line constituting our low income group. Taking 52% of the total population gives me 73,547.
Using Taro Yamane formula to arrive at the sampling size. The calculation of the sample size is as shown below.
Therefore our sample size is 207 low income earners that will be drawn randomly from Abakaliki metropolises.
References
BigBen, O (2016). Practical guide to social science research methodology. Ibadan, Nigeria: Winpress Publishing.
Dey, I. (1999). Grounding grounded theory: Guidelines for qualitative inquiry. San Diego, CA: Academic Press.
Hitchcock, J. H, Nastasi, B. K., Dai, D. C., Newman, J., Jayasena, A., Bernstein-Moore, R., Sarkar, S., & Varjas, K. (2004). Illustrating a mixed-method approach for identifying and validating culturally specific constructs. Accepted for publication in Journal of School Psychology.
Nastasi, B.K., Moore, R. B., & Varjas, K. M. (2004). School-Based Mental Health Services: Creating Comprehensive
