Cognitive interviewing is used to explore respondents' understanding and interpretation of survey questions and response frames. It is an important (but often overlooked) element of high quality survey design, which helps to reduce measurement error in quantitative research. Specialist researchers from our Qualitative Research Unit conduct face-to-face interviews with respondents to 'test' questions using 'think aloud' and probing techniques to identify respondents' comprehension, information retrieval, judgement and question response.