A base rate, or prevalence, is the proportion of a population with a specified condition. It supplies the prior probability before additional evidence is known. section 13.1
Table 13.1 A survey of 10,000 respondents organized in a marginal table
| Cold: YES |
Cold: NO |
Total |
|
|---|---|---|---|
| Sneezed: YES |
860 |
270 |
1,130 |
| Sneezed: NO |
140 |
8,730 |
8,870 |
| Total |
1,000 |
9,000 |
10,000 |
The survey table shows cold prevalence as 1,000 of 10,000 people, or 10%, alongside sneeze counts. section 13.1
A rare condition and a positive test
Frob Syndrome affects 1 in 30,000 people. Even with a test that has 99.5% sensitivity and specificity, a positive result yields only a 0.7% posterior probability in this example. Test accuracy alone does not determine the chance that a positive result is correct; prevalence matters. section 13.4
