Absolute frequencies

A 2x2-contingency table of observed absolute frequencies shows the 4 possible constellations of events:

	Disease present (D+)	Disease absent (D-)
Test result positive (T+)	A (events D+ and T+)	C (events D- and T+)
Test result negative (T-)	B (events D+ and T-)	D (events D- and T-)
	A + B (diseased subjects)	C + D (healthy subjects)

Now, computation of pre-test probability (P(D), post-test-probability P(D+/T+), sensitivity (P(T+/D+) and specificity P(T-/D-) is easy:

P(D)	(A + B) / (A + B + C + D)
P(D+/T+)	A / (A + C)
Sensitivity P(T+/D+)	A / (A + B)
Specificity P(T-/D-)	D / (C + D)