What is the relationship between reliability and validity?

The relationship between reliability and validity is foundational in psychometrics. A test cannot be valid if it is not reliable because reliability refers to the consistency of the test results over time or across different instances. If a test yields inconsistent results, it cannot accurately measure what it intends to measure, making it inherently invalid.

For example, if a scale were to give different weight readings for the same object, we cannot trust it to provide an accurate weight (validity), regardless of whether the measurements fall within an expected range. Thus, reliability is a prerequisite for validity. If a test is unreliable, any conclusions drawn from it would be questionable, leading to a misrepresentation of the construct being measured.

Moreover, while validity encompasses various aspects, including construct validity and content validity, its presence is contingent upon the reliability of the measures used. In other words, when a test is reliable, it establishes a foundation upon which validity can be evaluated.