The primary reason an organization will perform a job analysis is so that they can ensure that their employee selection procedure is valid and defensible. Practically speaking, a valid selection procedure is one that accurately measures the actual requirements of the job in a fair and reliable way. A valid selection procedure should use those duties and KSAPCs identified by the job analysis as being essential requirements for the position in question and use them as the basis for that selection procedure . Essentially, a valid selection procedure should effectively measure the net qualifications that are really needed for the job, and not much more or less.
In the legal realm, a selection procedure is valid if it can be proven by an employer in litigation that it is "... job related and consistent with business necessity" in order to address the requirements of the 1991 Civil Rights Act. This standard is usually met (or not) by arguing how the selection procedure first addresses the Uniform Guidelines, followed by professional standards, then by parallel or lower courts that have applied the standard in various settings.
Academically speaking, the Principles and Standards have adopted the same definition for validity: "The degree to which accumulated evidence and theory support specific interpretations of test scores entailed by proposed uses of a test." reference :