Modern validity theory is considered unitary and can be traced back to Lee Cronbach. In contrast to modern validity theory, older validity theory described different kinds of validity: content validity, construct validity, and criterion validity. Modern validity theory posits that all validation is singly focused on providing evidence to support the interpretation or the inference. All validation bears on validation of the claims or the inferences we want to make with respect to the construct; all validity is essentially construct validity.
Instead of discussing different kinds of validity, we now focus on potential sources of evidence to support the inference. The most relevant source of evidence is that which is directly tied to the purpose of the inference. For example, in the case of a typical standards–referenced test, the objective is to make a claim about some set of content standards, or some set of knowledge or skills.