Table 2.

The effect of prevalence on the predictive value of a screening test.

	Prevalence
	Low (6%)  screening∗			Moderate  (30%)  case-finding∗∗			High   (80%)  diagnosis∗∗∗
Test	Definitive diagnosis of condition
	Present	Absent	Total	Present	Absent	Total	Present	Absent	Total
Abnormal	48	94	142	240	70	310	640	20	660
Normal	12	846	858	80	630	690	160	180	340
Total	60	940	1000	300	700	1000	800	200	1000
	Predictive value of an abnormal test
	$\frac{48}{142}\times 100=34\%$			$\frac{240}{310}\times 100=77\%$			$\frac{640}{666}\times 100=97\%$
Hypothetical sensitivity = 80%, specificity = 90%

 ^∗All people on a family health roster.  ^∗∗All people in a memory disorder clinic.  ^∗∗∗All people in a locked unit in a long-term care facility.