How to Determine Sample Size for a Qualitative Doctoral Dissertation

How to Determine Sample Size for a Qualitative Doctoral Dissertation

Determining the sample size in a qualitative doctoral dissertation is not based on statistical formulas or power calculations. Instead, it is guided by the depth of data needed, the research design, and the concept of data saturation or information power.

In qualitative research, the goal is not to generalize to a population, but to develop a deep, detailed understanding of a phenomenon—such as lived experiences, perceptions, or social processes.

In simple terms, qualitative sample size is determined by: “How many participants are needed to fully understand the research phenomenon?”

What Is Sample Size in Qualitative Research?

The sample size refers to the number of participants included in a qualitative study.

Unlike quantitative research, where larger samples improve statistical validity, qualitative research focuses on:

• Depth of insight

• Richness of data

• Complexity of participant experiences

Typical qualitative samples can range from:

• 5–15 participants (phenomenology)

• 10–30 participants (case study or grounded theory)

• Larger samples in mixed qualitative designs

Key Principle: Data Saturation

The most common concept used to determine qualitative sample size is data saturation.

Data saturation means:

The point at which no new themes, insights, or information emerge from the data.

When additional interviews or responses begin to repeat what has already been found, saturation is likely reached.

Other Modern Concept: Information Power

Some researchers now use information power instead of saturation.

This means:

The more relevant and rich the data from each participant, the fewer participants are needed.

Information power depends on:

• Study aim

• Sample specificity

• Use of established theory

• Quality of dialogue or responses

• Type of analysis

• Data saturation for thematic analyses

Factors That Influence Qualitative Sample Size

Several factors determine how many participants you need in a doctoral dissertation.

1. Research Design

Different qualitative approaches require different sample sizes:

• Phenomenology: 5–15 participants

• Grounded theory: 15–30+ participants

• Case study: 1–10 cases (with multiple data sources)

• Narrative inquiry: 1–10 participants

2. Depth of Data Collection Method

• In-depth interviews: Smaller sample sizes

• Focus groups: Moderate sample sizes

• Open-ended questionnaires: May require slightly larger samples

• Observations + interviews (triangulation): Often fewer participants needed

3. Study Complexity

More complex research questions may require:

• More participants

• More diverse perspectives

• Multiple settings or contexts

4. Homogeneity vs. Heterogeneity of Participants

• Homogeneous groups (similar participants): Smaller sample sizes needed

• Heterogeneous groups (diverse participants): Larger sample sizes needed

5. Researcher Access and Feasibility

Practical considerations include:

• Time constraints

• Access to participants

• Ethical limitations

• Data collection resources

Step-by-Step: How to Determine Sample Size in a Qualitative Dissertation

Step 1: Identify Your Qualitative Methodology

Your design strongly influences sample size.

Example:

• Phenomenology → smaller sample (focus on lived experience)

• Grounded theory → larger sample (theory development)

Step 2: Define the Depth of Data Needed

Ask:

• Do I need rich, detailed narratives?

• Or broader perspectives across groups?

More depth per participant = fewer participants needed.

Step 3: Use Data Saturation as a Guide

During data collection:

• Continue interviewing until no new themes emerge

• Monitor repetition in responses

• Document when saturation is reached

Step 4: Consider Information Power

Evaluate:

• Are participants highly relevant to the research question?

• Is the interview guide focused and clear?

• Is the study narrow or broad in scope?

Higher information power = smaller sample size.

Step 5: Review Prior Literature

Look at similar qualitative dissertations:

• Identify common sample sizes

• Justify your decision based on precedent

Step 6: Justify Sample Size in Your Dissertation

In Chapter 3, clearly explain:

• Why your sample size is appropriate

• What concept guided your decision (saturation or information power)

• How you monitored data adequacy

Example Justification Statement:

The sample size of 12 participants was determined based on the principle of data saturation, which was reached when no new themes emerged during data analysis.

Step 7: Be Flexible During Data Collection

Qualitative research is iterative:

• You may adjust sample size as themes develop

• You may stop earlier or later than planned

• Decisions should be guided by data quality, not numbers alone

Common Sample Size Ranges in Qualitative Doctoral Research

Methodology	Typical Sample Size
Phenomenology	5–15 participants
Grounded Theory	15–30+ participants
Case Study	1–10 cases
Narrative Inquiry	1–10 participants
Qualitative Descriptive Study	10–30 participants

Common Mistakes When Determining Sample Size

Avoid:

• Using statistical formulas (not appropriate for qualitative research)

• Choosing arbitrary numbers without justification

• Ignoring saturation or information power

• Fixing sample size too early without flexibility

• Over-sampling without analytical depth

Strengths of Small Qualitative Sample Sizes

• Allows deeper engagement with participants

• Produces richer data

• Supports detailed thematic analysis

• Enhances validity through depth rather than breadth

Final Thoughts on How to Determine Sample Size for a Qualitative Doctoral Dissertation

For my qualitative case study, I initially wanted 15 to 20 participants. Then, I lowered this number to 10. Howver, I found that I had thematic saturation at 9 participants and having a 10th didn't necessarily add anything to my research in terms of information gain. Determining sample size in a qualitative doctoral dissertation is not about numbers alone—it is about depth, meaning, and the point at which new insights stop emerging. Whether guided by data saturation or information power, the goal is to ensure that your sample is sufficient to fully explore the research phenomenon.

A strong qualitative dissertation clearly justifies sample size based on methodology, research design, and evidence of saturation, rather than statistical calculation.

If you need help selecting a methodology, consider qualitative dissertation editing! If you need help with your Chapter 3, please visit our website.