Quality of Measurement

EPI9-4: THE QUALITY OF MEASUREMENTS: How Does One Test For Validity?

OBJECTIVES

At the end of this session you should know how the validity of a screening test is assessed making use of the 2 by 2 table.

TESTING VALIDITY:

One can test for criterion validity by setting up 2 by 2 table as in Table 1 1 below where we know the truth and test the results of a screening test against this truth. For example, say that all subjects with TB were sputum smear and/or sputum culture positive for M. tuberculosis and that this represented the "truth" as to whether they had TB or not. Performing a chest x-ray is a screening test for pulmonary TB and the results are expressed in algebraic form in the table.

The performance of chest x-rays as a screening tool for TB is described by its:

sensitivity = a/N₁
specificity = d/N₀
positive predictive value = a/M₁
negative predictive value = d/M₀

**Table 1: The screening test is examined against a gold standard for the truth.**
	The truth +	The truth -
Screening test +	a	c	M₁
Screening test -	b	d	M₀
	N₁	N₀	T

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License.