E-Epidemiology
Positive predictive value
To assess this positive and negative predictive values are used. These measures answer the following questions:
Given a positive test result what is the probability that the individual is truly sick (infected)?
This is evaluated by the test positive predictive value:
| Positive predictive value PPV = probability P (Disease + | Test +) |
In our contingency table this would be: a / (a+b), with cell a showing the number of true positive animals and the sum of cells a and b referring to all positive animals.
Thus, the positive predictive value represents the proportion or percentage of individuals who test positive who are actually sick.
| Disease + |
Disease - | Total | |
| Test + |
a |
b |
a + b |
| Test - | c |
d |
c + d |
| Total | a + c |
b + d | a + b + c + d |