How to Determine Sample Size for a Qualitative Psychology Doctoral Dissertation

How to Determine Sample Size for a Qualitative Psychology Doctoral Dissertation

Determining the sample size for a qualitative psychology doctoral dissertation is one of the most important methodological decisions in the research design, but it is often misunderstood. Unlike quantitative research, qualitative studies do not rely on statistical formulas or power analysis. Instead, sample size is guided by depth of understanding, data richness, and the point at which no new psychological insights emerge.

In qualitative psychology research, the goal is not to generalize findings statistically, but to understand lived psychological experiences, meanings, and processes in depth.

In simple terms, qualitative sample size answers: “How many participants are needed to fully understand the psychological phenomenon being studied?”

What Is Sample Size in Qualitative Psychology Research?

The sample size refers to the number of participants included in a qualitative study such as:

• Phenomenological studies

• Grounded theory studies

• Narrative inquiry

• Qualitative case studies

• Thematic analysis-based designs

In psychology, qualitative sample sizes are typically smaller but richer in detail, focusing on:

• Emotional experiences

• Cognitive interpretations

• Behavioral meaning-making

• Psychological processes

Key Principle: Data Saturation

The most widely used concept for determining sample size is data saturation.

Data saturation means:

The point at which additional data collection no longer produces new themes, insights, or psychological meanings.

Once participants begin repeating similar ideas, experiences, or themes, saturation is considered reached.

Example in psychology:

If interviewing students about anxiety:

• Early interviews reveal multiple new emotional patterns

• Later interviews repeat similar themes like fear, avoidance, and stress

• When no new psychological themes emerge → saturation is achieved

Alternative Concept: Information Power

A more modern approach used in psychology is information power.

Information power means:

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

Sample size depends on:

• Study aim specificity

• Participant relevance

• Quality of dialogue or responses

• Use of established psychological theory

• Depth of analysis

Factors That Influence Sample Size in Qualitative Psychology

Several key factors determine how many participants you need.

1. Research Design

Different qualitative approaches require different sample sizes:

• Phenomenology: 5–15 participants

• Narrative inquiry: 1–10 participants

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

• Grounded theory: 15–30+ participants

• Generic qualitative study: 10–25 participants

2. Depth of Psychological Data Collection

The more in-depth the method, the smaller the sample size needed.

Examples:

• In-depth interviews → fewer participants

• Focus groups → moderate sample sizes

• Open-ended questionnaires → may require slightly larger samples

3. Homogeneity of Participants

Homogeneous sample (similar experiences):

• Smaller sample size needed

• Example: graduate students with academic burnout

Heterogeneous sample (diverse experiences):

• Larger sample size needed

• Example: adults from different clinical backgrounds

4. Complexity of the Psychological Phenomenon

More complex psychological topics may require:

• More participants

• More varied perspectives

• Multiple data sources

5. Researcher Access and Feasibility

Practical limitations also matter:

• Time constraints

• Access to clinical or student populations

• Ethical approval limitations

• Recruitment feasibility

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

Step 1: Define the Psychological Phenomenon

Clearly identify what you are studying.

Examples:

• Anxiety experiences in college students

• Burnout among healthcare workers

• Trauma recovery processes

• Identity development in adolescents

• Coping strategies in chronic illness

Step 2: Select a Qualitative Methodology

Your methodology influences sample size expectations.

Common psychology methodologies:

• Phenomenology → lived experience (small sample)

• Grounded theory → theory development (larger sample)

• Narrative inquiry → life stories (very small sample)

Step 3: Decide on Data Collection Method

Different methods affect saturation:

• In-depth interviews → smaller samples

• Written qualitative questionnaires → slightly larger samples

• Focus groups → moderate samples

Step 4: Use Data Saturation as the Primary Guide

During data collection:

• Monitor repetition of psychological themes

• Track when no new insights emerge

• Document saturation decision clearly

Step 5: Consider Information Power

Ask:

• Are participants highly relevant to the research question?

• Is the phenomenon narrowly defined?

• Is the interview guide focused and structured?

Higher information power = smaller sample size needed.

Step 6: Review Similar Psychology Dissertations

Look at prior research:

• Identify common sample sizes

• Justify your choice using methodological precedent

Step 7: Justify Sample Size in the Dissertation

In Chapter 3, clearly explain:

• Why the sample size is appropriate

• What guided the decision (saturation or information power)

• How you determined when to stop recruiting

Example Justification:

The sample size of 12 participants was determined using the principle of data saturation. Data collection continued until no new psychological themes emerged from participant interviews.

Typical Sample Size Ranges in Qualitative Psychology Research

Methodology	Sample Size Range
Phenomenology	5–15 participants
Narrative inquiry	1–10 participants
Case study	1–10 cases
Grounded theory	15–30+ participants
Qualitative descriptive studies	10–25 participants

Common Mistakes in Determining Sample Size

Avoid:

• Using statistical formulas (not appropriate for qualitative research)

• Fixing sample size too early without flexibility

• Ignoring data saturation

• Choosing arbitrary numbers without justification

• Over-recruiting without added analytical value

Strengths of Smaller Qualitative Psychology Samples

• Deeper psychological insight

• Rich, detailed narratives

• Strong thematic development

• Better understanding of lived experience

• Higher-quality interpretation

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

Determining sample size in a qualitative psychology doctoral dissertation is not about numbers alone—it is about achieving depth, meaning, and conceptual completeness. The most widely accepted approach is data saturation, supported by information power and methodological justification.

A strong qualitative psychology dissertation clearly explains how and why sample size was determined, demonstrating that enough participants were included to fully understand the psychological phenomenon being studied.

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