The devil is in the detail: reflections on the value and application of cognitive interviewing to strengthen quantitative surveys in global health.

Cognitive interviewing is a qualitative research method for improving the validity of quantitative surveys, which has been underused by academic researchers and monitoring and evaluation teams in global health. Draft survey questions are administered to participants drawn from the same population as the respondent group for the survey itself. The interviewer facilitates a detailed discussion with the participant to assess how the participant interpreted each question and how they formulated their response. Draft survey questions are revised and undergo additional rounds of cognitive interviewing until they achieve high comprehension and cognitive match between the research team's intent and the target population's interpretation. This methodology is particularly important in global health when surveys involve translation or are developed by researchers who differ from the population being surveyed in terms of socio-demographic characteristics, worldview, or other aspects of identity. Without cognitive interviewing, surveys risk measurement error by including questions that respondents find incomprehensible, that respondents are unable to accurately answer, or that respondents interpret in unintended ways. This methodological musing seeks to encourage a wider uptake of cognitive interviewing in global public health research, provide practical guidance on its application, and prompt discussion on its value and practice. To this end, we define cognitive interviewing, discuss how cognitive interviewing compares to other forms of survey tool development and validation, and present practical steps for its application. These steps cover defining the scope of cognitive interviews, selecting and training researchers to conduct cognitive interviews, sampling participants, collecting data, debriefing, analysing the emerging findings, and ultimately generating revised, validated survey questions. We close by presenting recommendations to ensure quality in cognitive interviewing.