How does the theory of validity relate to clinical evidence in the context of nursing diagnoses?

The theory of validity asks whether a nursing diagnosis's defining characteristics actually measure the diagnosis they represent, while clinical evidence provides graded research support for that claim. NANDA-I connects them by setting levels of evidence for diagnostic validity based on the types of studies that generated the diagnosis.

In the context of nursing diagnoses, the theory of validity, originally developed for measurement instruments, states that a valid instrument measures what it proposes to measure. Applied here, a nursing diagnosis is valid when its defining characteristics measure the diagnosis they are supposed to represent. Clinical evidence enters because most human responses cannot be directly observed and no gold standard exists for most nursing diagnoses, so proof that defining characteristics represent a diagnosis must come from research evidence. Evidence is a continuum of weaker to stronger support, and NANDA-I has created a hierarchy of evidence for diagnostic validity guided by the types of studies that generated the diagnoses. Thus, clinical evidence is the empirical basis for judging how well the defining characteristics actually measure the diagnosis, which is exactly what validity theory asks.