OBJECTIVES: As the role and importance of patient-reported outcomes (PROs) increase, the validity and reliability of PRO measures come under greater scientific and regulatory scrutiny. One key issue is selecting the 'most appropriate' recall period for capturing PROs in clinical trials. This paper draws on survey research, health-specific literature, and results from clinical trials to summarize factors that can influence recall and provide guidance on selecting an optimal recall period. METHODS: We conducted a systematic review of six databases and additional literature drawn from bibliographies of the selected articles. RESULTS: Six major factors can influence recall; these can be classified into two broad areas: characteristics of the recalled phenomenon (recency, attributes, complexity) and context or meaning of the recalled phenomenon (salience, patient experience, mood). Results of different recall periods for three classes of PROs are presented: health behaviors, symptoms, and health-related quality of life. We present findings on the effect of alternative recall periods for three commonly used PROs. Finally, we propose a heuristic model to link the concept under investigation with an optimal recall period. CONCLUSIONS: No single recall period is best for all measures or all phenomena. The recall period must correspond to the characteristics of the phenomenon of interest and the purpose of the assessment. Recall period is an issue of internal validity. An incorrect recall period introduces measurement error that may reduce the chances of detecting a treatment effect. Researchers should consider recall period as seriously as they do other measurement properties.
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