An imperfect test can also provide false positive results, i.e. a positive test from an healthy animal.

This is described by the diagnostic test specificity = probability of having a negative test result among non-infected animals:

Specificity (SP) = probability P (Test- | Disease-)

 

 

 

In our contingency table this would be: d / (b + d) with cell d showing the number of true negative animals and the sum of cells b and d referring to all healthy animals.

	Disease +	Disease -	Total
Test +	a	b	a + b
Test -	c	d	c + d
Total	a + c	b + d	a + b + c + d