This is particularly important with achievement tests.
Consider that a test developer wants to maximize the validity of a unit test for 7th grade mathematics.
Consider the SAT, used as a predictor of success in college.
It is a reliable test (high scores relate to high GPA), though only a moderately valid indicator of success (due to the lack of structured environment – class attendance, parent-regulated study, and sleeping habits – each holistically related to success).
It is rare, if nearly impossible, that an instrument be 100% valid, so validity is generally measured in degrees.
As a process, validation involves collecting and analyzing data to assess the accuracy of an instrument.
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There are numerous statistical tests and measures to assess the validity of quantitative instruments, which generally involves pilot testing.
The remainder of this discussion focuses on external validity and content validity. Establishing eternal validity for an instrument, then, follows directly from sampling.