Which statement best differentiates reliability and validity in assessment, with a PT example?

• A. Reliability = consistency; Validity = measures intended construct.
• B. Reliability = speed of measurement; Validity = cost-effectiveness.
• C. Reliability = accuracy; Validity = repeatability.
• D. Reliability = validity; Validity = reliability.

Answer: (D)

Reliability and validity are two separate properties of any assessment. Reliability is about consistency—if you repeat the measurement under the same conditions, you should get similar results. Validity is about accuracy for the intended purpose—the test must actually measure what you intend to measure.

In physical therapy, think of a functional mobility test used to guide rehab. If you administer it on two different days and the results are nearly the same, the test shows reliability. If the same test truly reflects the patient’s functional mobility and relates appropriately to other indicators of mobility (like gait speed or balance tests), it demonstrates validity. It’s possible to have a test that is reliable but not valid if it gives consistent scores that don’t reflect the intended construct, so validity depends in part on reliability, but they are not the same thing.

The statement that reliability means consistency and validity means measuring the intended construct best captures the standard definitions. The other options describe aspects like speed, cost, or mix up the ideas, which do not correctly differentiate reliability and validity.