Multiple assessment in quality of life trials: how many questionnaires? How often should they be given?

Researchers conducting randomized trials of integrative interventions often ask patients to complete multiple different quality of life questionnaires or symptom severity scales repeatedly over the course of a trial. Although trialists rarely if ever give a strong justification for either the number of questionnaires they give or how often they give them, these are design decisions that can be taken systematically. Giving large numbers of questionnaires can improve the precision of trial results, and provide interesting secondary data, such as on the time course of symptoms. However, doing so can also lead to excessive patient drop-out, an undue data management burden and difficulties with interpretation of results. As a general guideline, each aspect of quality of life should be measured by a single questionnaire, and researchers should avoid giving more than three different questionnaires to patients. Decisions about the appropriate number of assessments to use can be based on statistical properties derived from simple formulae.