Students are often confused by two other ratios that an eval-uator can compute from the data in Table 20.1. The world would be a far less confusing place if these ratios did not exist at all. We will not discuss them further because they are rarely useful, but for the record, they are as follows:
Predictive value positive (PVP): the fraction of the total number of cases that were classified as SARS that truly were SARS.
Predictive value negative (PVN): the fraction of the total number of cases that were classified as non-SARS that truly were non-SARS.
Readers who wish to learn more about PVP and PVN and the reason why we discourage their use can refer to Appendix B.
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