OBJECTIVES: Qualitative interviews conducted within clinical trials are becoming increasingly popular, but guidance around adequate sample size is lacking. To address this gap, the current research summarizes sample sizes from published in-trial qualitative interview studies and considerations for determining sample size in this context.

METHODS: A literature search was conducted to identify qualitative in-trial interview studies published in 2012-2022. Publications were reviewed for relevance to the study objective and selected for inclusion if the interviews were conducted within trials evaluating a medical treatment (excluding behavioral-based interventions), with trial participants and/or caregivers, and related to patient experiences in the trial.

RESULTS: A total of 168 publications were identified; ten articles met criteria for full-text review. Interviews were conducted in Phase 2 or 3 clinical trials across seven different therapeutic areas and primarily aimed to better understand changes in disease experience during the trial, treatment experience, and/or to inform measurement strategy. Clinical trial sample size ranged from N=23 to 1,879, while interview sample size ranged from N=11 to 90 (mean≈40). Most studies (n=6) targeted a subset of trial participants to interview, which made up 2–26% of the total trial population. Considerations for limiting the number of interviews were reportedly related to logistics or interview objective. Four studies aimed to interview all trial participants, and 48–92% of each total trial population ultimately participated in an interview. Missed interviews were reportedly related to trial attrition or lack of interest from the participant or site. Three studies (N=12–28 participants) reported evaluating and achieving concept saturation to confirm the adequacy of the interview sample size.

CONCLUSIONS: This review identified a range of sample sizes and sampling considerations for in-trial interviews. Selecting an appropriate interview sample size within clinical trials should be guided by interview objectives and trial design, while considering any methodological limitations.