Question

Suppose a test for a disease has a sensitivity of 80% and a specificity of 95%. Further suppose that in a certain country with a population of 1,000,000, 21% of the population has the disease. Fill in the accompanying table.

-------------------Has disease |Does not have disease| Totals

Test positive

Test negative

Totals

Answer 1

	Has Disease	Does not have Disease	Totals
Test positive	A	B	C
Test negative	D	E	F
Totals	G	H	1,000,000

A is the sensitivity % of disease total and E is the specificity % of Does not have disease total

21% of population has the disease

this means, G = 21% of grand total = (21/100)*1,000,000 = 210,000

And A is 80% of G = (80/100)*(G value) = 0.80*210,000 = 168,000

Therefore, D = G-A = 210,000 -168,000 = 42,000

Again

H = Grand total - G

= 1,000,000- (number of people who has disease

= 1,000,000 - 210,000

= 790,000

E is the specificity % of Does not have disease total

this implies, E = 95% of H = (95/100)*790,000 = 750,500

And B = H - E = 790,000 - 750,500 = 39,500

Last column calculation

C = A+B = 168,000 + 39,500 = 207,500

F = D +E = 42,000 + 750,500 = 792,500

So, we get the following table

	Has Disease	Does not have Disease	Totals
Test positive	168,000	39,500	207,500
Test negative	42,000	750,500	792,500
Totals	210,000	790,000	1,000,000