Question

Suppose that your company has just developed a new screening test for a disease and you are in charge of testing its validity and feasibility. You decide to evaluate the test on 1000 individuals and compare the results of the new test to the gold standard. You know the prevalence of disease in your population is 30%. The screening test gave a positive result for 292 individuals. Two hundred eighty-five (285) of these individuals actually had the disease on the basis of the gold standard determination.

1. Fill in all cells of the two by two table. (5 points)

		Gold Standard Determination of Disease		Total
Results of Screening Test		Yes	No
	Positive
	Negative
	Total

b. Calculate and interpret the predictive value of a positive test. (5 points)

c. What would happen to the predictive value positive if this test were administered in a population with a disease prevalence of 1% instead of 30%? (Note that the sensitivity and specificity of the test remain the same.) Would it remain the same? Increase? Decrease? (5 points)

Answer 1

a.

	Gold Standard Determination of Disease		Total
	Yes	No
Positive	285	7	292
Negative	15	693	708
Total	300	700	1000

b. Positive predictive value = True positive / (True positive + False positive)

= 285 / 292 = 0.97

c. Would decrease as prevalence is decreased from 30 % to 1 %

Positive predictive value would increase when prevalence increases and it would decrease when prevalence decreases. Both are directly proportional.