Dissertation Chapter 3: Justifying a Smaller Sample Size

A smaller than planned sample does not automatically invalidate a dissertation. Its implications depend on the research design, the purpose of the study, the analysis being conducted, and the conclusions the available data can support.

The challenge is explaining that position in Chapter 3. Your methodology chapter must show why the sample is appropriate for the research, what happened if recruitment fell short of the original target, and how the final sample affects the study's limitations.

A defensible justification does not simply explain why fewer participants were recruited. It demonstrates that the researcher understands the methodological consequences and has made appropriate decisions in response.

1. Start With the Original Sample Size Rationale

Before explaining why your final sample was smaller, revisit how you established the original target.

Your proposal should identify the reasoning behind the intended sample size. That reasoning may have come from a power analysis, the requirements of a qualitative research design, methodological literature, comparable studies, or a combination of these considerations.

Your explanation should answer three questions:

How was the original sample size determined?
What assumptions or methodological requirements informed that decision?
How does the final sample compare with the original target?

For example, a quantitative study might have established its target through an a priori power analysis based on the anticipated effect size, significance level, statistical power, and planned analysis. A qualitative study might have justified its proposed sample through the research approach, the characteristics of the participants, the depth of the intended interviews, and the anticipated information needed to address the research question.

These rationales are not interchangeable. Your justification must reflect the methodology you actually used.

2. Explain Why the Final Sample Was Smaller

State the difference between the planned and achieved sample clearly. Then explain the circumstances that produced it.

Depending on the study, these circumstances might include recruitment difficulties, participant withdrawal, limited access to the target population, eligibility restrictions, or a lower response rate than anticipated.

Avoid relying on a general statement that recruitment was difficult. Explain what happened and how it affected the final sample.

For example, if you planned to recruit 150 eligible participants but obtained 92 usable responses, report those figures and explain the recruitment circumstances. If some responses were excluded, distinguish the number recruited from the number included in the final analysis and explain the exclusion criteria.

The explanation should remain factual. Recruitment difficulties provide context, but they do not, by themselves, demonstrate that the final sample is methodologically adequate.

3. Evaluate What the Smaller Sample Means for Your Research Design

The most important question is not simply whether you reached your target. It is whether the final sample can support the analysis and conclusions your study intends to produce.

The answer differs between qualitative and quantitative research.

Qualitative research

Qualitative sample adequacy depends on the research tradition, the study's purpose, the specificity of the research question, participant characteristics, the quality and depth of the data, and the analytical approach.

Researchers sometimes use saturation to justify qualitative sample size. However, saturation is not a universal requirement across all qualitative methodologies, and it should not be claimed merely because interviews stopped producing obviously new observations.

Where saturation is relevant to the chosen approach, explain how it was assessed and what evidence supports the conclusion. Other qualitative approaches may use different concepts of adequacy, such as information power or the depth and relevance of the material collected.

A smaller qualitative sample may still produce meaningful findings when participants have direct experience of the phenomenon and the data provide sufficient depth to address the research question. Nevertheless, a small sample does not guarantee depth, just as a larger sample does not guarantee it.

In Chapter 3, connect your final sample to the actual design and analytical process. Explain why the data were suitable for the intended analysis and acknowledge any remaining limitations.

Quantitative research

In quantitative research, the implications of a smaller sample depend on the statistical analysis and the assumptions underlying the study.

A sample below the planned target may reduce statistical power, increase uncertainty around estimates, and make it more difficult to detect an effect if one exists. The extent of the problem depends on factors such as the expected effect size, variability, significance level, study design, and analysis.

If your original target was established through an a priori power analysis, compare the achieved sample with that target. Explain whether the assumptions behind the original calculation remain appropriate and how the smaller sample affects the interpretation of the results.

Do not use a post hoc power calculation as proof that the completed study had adequate power. Instead, focus on the observed estimates, confidence intervals where appropriate, effect sizes, and the uncertainty surrounding the findings.

A smaller quantitative sample may still provide useful evidence, but it may not support the same level of precision or the same conclusions anticipated in the proposal.

Mixed-methods research

For mixed-methods studies, assess sample adequacy separately for each component. The requirements for the quantitative strand may differ substantially from those for the qualitative strand.

Explain how the final samples affect each component and whether the study can still integrate the findings as originally intended. A defensible justification should not treat the overall participant count as sufficient evidence that both strands are adequately supported.

4. Address Any Departure From the Approved Proposal

If your achieved sample differs materially from the number or recruitment procedure approved in your proposal, explain the change and how it was handled.

Describe the original plan, the actual procedure, and the reason for the departure. Where required, identify the steps taken to obtain approval from your supervisor, committee, ethics body, or other relevant authority.

Do not present a substantial methodological change as a minor administrative adjustment if it affects the study's design or interpretation.

For example, a researcher who changes recruitment methods because the original population is inaccessible may need to explain whether the revised approach introduces selection bias or changes the population to which the findings can reasonably apply.

The appropriate response depends on the nature of the change and your institution's requirements. Consult your supervisor or committee when a departure affects the approved methodology.

5. Write the Justification Into Chapter 3

Your explanation should appear where readers expect to find the sample size rationale and sampling procedures. The exact headings depend on your institutional template, but the discussion commonly belongs in the sampling, participant selection, or sample size subsection.

A clear explanation usually covers the following:

Planned sample: State the intended sample size and the rationale behind it.
Achieved sample: Report the final number of participants, cases, or usable responses included in the analysis.
Recruitment outcome: Explain why the final number differed from the target.
Methodological implications: Discuss what the difference means for the design, analysis, and interpretation.
Limitations: Identify the conclusions or claims that need to be qualified as a result.

Use the figures and methodological rationale specific to your study. Avoid inserting generic statements about sample adequacy that are not supported by your research design or data.

For instance, writing that the sample was sufficient because it was consistent with previous studies is not enough on its own. You must explain why those studies provide a relevant comparison and whether their populations, research questions, designs, and analytical methods are comparable to yours.

Similarly, stating that the sample was adequate because it met the minimum requirement cited in a textbook may be insufficient if that requirement does not fit your particular design.

6. Recognize the Limits of Your Findings

A defensible methodology chapter does not attempt to make every limitation disappear. It explains how the limitations affect the study and what conclusions remain reasonable.

Depending on the research, a smaller sample may limit statistical precision, reduce the range of perspectives represented, constrain comparisons between groups, or narrow the transferability of qualitative findings.

Be specific about the implications for your study. Avoid claiming that a smaller sample necessarily makes findings unreliable or that a carefully justified sample eliminates all concerns.

Your explanation should distinguish between what the data support, what they suggest, and what remains uncertain. If the sample limits the scope of your conclusions, state that directly.

This alignment must continue into Chapter 4 and Chapter 5. Your results should reflect the evidence collected, while your discussion should acknowledge the implications of the final sample for the study's conclusions and recommendations.

7. Check the Chapter Before Submission

Before submitting Chapter 3, verify that:

The original sample size rationale is clear and appropriate to the research design.
The planned and achieved sample sizes are reported accurately.
Recruitment outcomes and exclusions are explained transparently.
Claims about saturation, statistical power, or sample adequacy are methodologically justified.
Any material departure from the approved proposal has been addressed.
The limitations reflect the actual consequences of the final sample.
The methodology, analysis, findings, and conclusions remain aligned.
The discussion follows your university's dissertation guidelines and your committee's requirements.

A smaller sample is not automatically a methodological failure. The central issue is whether the final sample, the analysis, and the claims made in the dissertation are appropriately aligned.

Need a review of your Chapter 3 methodology?

The Two Dissertation Moms provides Research Design and Methodology Review for postgraduate researchers who need to assess their sampling rationale, research procedures, methodological alignment, and response to committee feedback. A review can help identify gaps in the explanation and clarify what needs attention before submission.