Qualitative sampling: seek information and disconfirming cases

Predict with context

Purposeful sampling selects cases for a question, such as variation, unusual experience or a proposed mechanism. A run of repeated themes in a narrow recruitment network may reveal similarity in that network rather than adequate coverage of the question.

Predict how the conclusion changes when the missing context or competing account is added. State your reason before reading feedback.

Inspect the authentic source

The CDC chapter describes qualitative samples as typically small and purposive. It discusses gathering information-rich accounts and assessing whether further interviews yield additional insights, rather than treating qualitative sampling as a simple population-frequency estimator.

Wolff, Mahoney, Lohiniva and Corkum; CDC Field Epidemiology Manual — source note. 8 August 2024. Authored paraphrase of an authentic official or educational source. This note is not a verbatim primary excerpt. Its description, purpose and limits must stay attached.

Worked investigation

Synthetic service study interviews six satisfied volunteers referred by one coordinator. All report convenient hours. A seventh, recruited from people who stopped attending, reports shift conflicts. That account challenges the emerging claim that hours are universally convenient; it does not reveal how common conflicts are.

Produce an inspectable result

Build a sampling matrix crossing participation status and work schedule. Select a next case for each unresolved explanation, and maintain a log of new themes, changed meanings and unanswered questions. Write a stopping rationale tied to scope rather than a magic interview count.

Keep the original record available alongside your analysis. Label invented details, attributed descriptions and your own interpretations separately. For numeric work, show the intermediate arithmetic and units; for interpretive work, point to the specific feature supporting a claim. A reader should be able to identify where another interpretation could enter.

Transfer

Ten interviews repeat the same phrase but participants mean different things by it. What should happen?

Write the answer and its reason before opening feedback. Identify one similarity that makes transfer useful and one difference that limits it. State a specific next observation rather than saying only that more information is needed.

Limits and responsibility

There is no universal sample size guaranteeing sufficient qualitative insight. Do not collect identifiable personal accounts for the exercise; use the supplied synthetic records and a proposed recruitment plan.

A source can be authoritative about its own procedure or announcement without establishing every outcome that someone might infer from it. Ask whether the record describes an action, expresses a principle, documents an observation or interprets another source. Those are different evidential roles.

Extend

High school: Complete the specified diagram, record or table and explain one assumption in plain language. All supplied evidence can be examined without collecting personal information.

College: Add a bounded authentic case or methodological record that tests the weakest assumption. Document version, access, selection and interpretation, and present a justified alternative conclusion.