The cognitive interviewing approach to evaluating sources of response error in survey questionnaires was developed during the 1980s through an interdisciplinary effort by survey methodologists and psychologists. It explicitly focuses on the cognitive processes that respondents use to answer survey questions; therefore, processes that are normally hidden, as well as observable processes, are studied. Although cognitive interviewing is a powerful tool, cognitive interviewing is practiced in relatively few places, mostly in federal statistical agencies and survey research organisations in North America and Europe.

There are two major sub-types of cognitive interviewing methods: think-aloud interviewing and verbal probing techniques. The term think-aloud is used here to describe a specific type of activity in which subjects are explicitly instructed to "think aloud" as they answer the survey questions. The interviewer reads each question to the subject, and then records and/or otherwise notes the processes that subject uses in arriving at an answer to the question. The interviewer interjects little else, except to say "tell me what you're thinking" when the subject pauses. As an alternative to the think-aloud, verbal probing is used. After the interviewer asks the survey question, and the subject answers, the interviewer then asks for other, specific information relevant to the question, or to the specific answer given. In general, the interviewer "probes" further into the basis for the response.

The two general approaches to probing are: concurrent probing and retrospective probing. With concurrent probing, the interchange is as follows: a) the interviewer asks the survey question; b) the subject answers the question; c) the interviewer asks a probe question; d) the subject answers the probe question; and e) possibly, further cycles of (c-d). In retrospective probing, the subject is asked the probe questions after the entire interview has been administered, sometimes in a separate part of the interview known as a “debriefing session.”

Cognitive interviewing can take place either in a particular "laboratory" or in location(s) similar to those used during the actual survey interviews. Although organisations that conduct relatively large numbers of cognitive interviews have dedicated laboratory facilities containing video and audio equipment, and remote observation capability, cognitive interviewing does not require special physical environments, or sophisticated recording equipment. Equipment needs are minimal. It is helpful to have a tape-recorder, as it is useful to record interviews (most subjects do not object, as long as privacy and confidentiality requirements are met).

1. This definition is adapted from Willis, G., Cognitive Interviewing: A How-To Guide, 1999. Further information on cognitive interviewing can be found in that resource.