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For goniometry to provide meaningful information, measurements must be valid and reliable. Currier1 states that validity is "the degree to which an instrument measures what it is purported to measure; the extent to which it fulfills its purpose." Stated in another way, the validity of a measurement refers to how well the measurement represents the true value of the variable of interest. The purpose of goniometry is to measure the angle created at a joint by the adjacent bones of the body. Therefore, a valid goniometric measurement is one that truly represents the actual joint angle. The joint angle is used to describe a specific joint position or, if a beginning and ending joint position are compared, a range of motion (ROM).

Face Validity

There are four main types of validity: face validity, content validity, criterion-related validity, and construct validity.2,3,4,5 Most support for the validity of goniometry is in the form of face, content, and criterion-related validity. Face validity indicates that the instrument generally appears to measure what it proposes to measure—that it is plausible.2,3,4,5 Much of the literature on goniometric measurement does not specifically address the issue of validity; rather, it assumes that the angle created by aligning the arms of a universal goniometer with bony landmarks truly represents the angle created by the proximal and distal bones composing the joint. One infers that changes in goniometer alignment reflect changes in joint angle and represent a range of joint motion. Portney and Watkins3 report that face validity is easily established for some tests, such as the measurement of ROM, because the instrument measures the variable of interest through direct observation.

Content Validity

Content validity is determined by judging whether or not an instrument adequately measures and represents the domain of content—the substance—of the variable of interest. 2,3,4,5 Both content and face validity are based on subjective opinion. However, face validity is the most basic and elementary form of validity, whereas content validity involves more rigorous and careful consideration. Gajdosik and Bohannon6 state, "Physical therapists judge the validity of most ROM measurements based on their anatomical knowledge and their applied skills of visual inspection, palpation of bony landmarks, and accurate alignment of the goniometer. Generally, the accurate application of knowledge and skills, combined with interpreting the results as measurement of ROM only, provide sufficient evidence to ensure content validity."

Criterion-Related Validity

Criterion-related validity justifies the validity of the measuring instrument by comparing measurements made with the instrument to a well-established gold standard of measurement—the criterion.2,3,4,5 If the measurements made with the instrument and criterion are taken at approximately the same time, concurrent validity is tested. Concurrent validity ...

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