Measurement validity in physical therapy research.

This article considers the role of measurement validity within physical therapy research. The concept of measurement validity is identified as a component of internal validity, and it is differentiated from the notion of reliability; these concepts are related to systematic and random sources of error, respectively. Using examples from physical therapy and rehabilitation, four main types of validity are reviewed: face validity, criterion-related validity, content validity, and construct validity. The differing implications of these types of validity for quantitative and qualitative research are discussed. Three principal areas of concern are then addressed, based on a critical discussion of selected examples from the literature. First, it is argued that validity is often poorly distinguished from the allied concept of reliability and that purported claims for validity often only demonstrate reliability. Second, it is claimed that validity is too often neglected in favor of reliability, and specific examples relating to gait analysis are put forward to support this argument. Third, some of the methodological difficulties that may occur when attempts are made to demonstrate validity are considered. The article concludes with a plea for a closer focus on the issue of measurement validity within physical therapy research.