The design and assessment of questionnaires in clinical research.

Questionnaires are one of the most commonly used tools for data collection in clinical research. Despite its simplicity and convenience of use, the design of questionnaire instruments that accurately measure health status and their determinants is nevertheless a difficult and challenging task.We review the two most important issues which are reliability and validity. Reliability can be defined as the degree to which a measure gives 'consistent' or 'reproducible' values when applied in different situations. Validity refers to the extent in which the true value of a variable is correctly measured by the instrument. For different types of questionnaire measurement instruments, specific issues of content, construct and criterion validity should be appropriately addressed. Accuracy in questionnaire-based measurement in clinical studies is achieved by paying attention to the relevant specific issues of reliability and validity during development and testing of such questionnaires.