Validity and accuracy

Validity

Validity describes whether the results of an experiment really do measure the concept being tested. In other words, it says something about the experiment (or study, or surveillance system) as a whole; the design, methods and tests included. If the design or the choice of methods is inappropriate in relation to the aim of the experiment, then the results will be considered not valid, even when the tests used have produced accurate measurements. Likewise, if the design and methods of s study have been chosen appropriately (regarding the aim of the study), then the validity of the results will be mostly determined by the accuracy of the tests used. Therefore the rest of this article will focus in more detail on the concept of accuracy.

Consistency in the production of good results requires a standardized operating procedure that includes quality assurance, quality control, and quality assessment [1].

Accuracy

The accuracy (performance) of a diagnostic test is expressed in four dimensions (sensitivity, specificity, positive predictive value and negative predictive value), The prevalence of the disease or condition tested for affects some - but not all - of the test performance characteristics [1].

		Does the person truly have the condition?
		YES	NO
Test result	Positive	A (true positive)	B (false positive)	A + B
	Negative	C (false negative)	D (true negative	C + D
		A + C	B + D	A + B + C + D

The sensitivity of a diagnostic test measures the proportion of those people who have the disease and are correctly detected by the test (test positive). The sensitivity of a test can only be measured among those for whom the diagnosis has already been confirmed by other means than the test under study.