TABLE 6.
Predictive accuracy of water quality indicators for virus occurrence in the same sample or in the same well
| Basis | Indicator | True-positive rate (%)a | True-negative rate (%)b | Positive predictive value (%)c | Negative predictive value (%)d |
|---|---|---|---|---|---|
| Per sample | Total coliforms | 20.0 | 93.1 | 7.1 | 97.8 |
| E. coli | 0 | 99.5 | 0 | 97.9 | |
| Fecal enterococci | 0 | 97.3 | 0 | 97.8 | |
| FRNA coliphages | 0 | 98.9 | 0 | 97.4 | |
| NO3 > 10 mg liter−1 | 0 | 95.2 | 0 | 97.3 | |
| Cl > 28 mg liter−1 | 60.0 | 69.5 | 5.0 | 98.5 | |
| Per well | Total coliforms | 25.0 | 71.7 | 7.1 | 91.7 |
| E. coli | 0 | 97.8 | 0 | 91.8 | |
| Fecal enterococci | 0 | 89.1 | 0 | 91.1 | |
| FRNA coliphages | 0 | 95.7 | 0 | 91.7 | |
| NO3 > 10 mg liter−1 | 0 | 91.3 | 0 | 91.3 | |
| Cl > 28 mg liter−1 | 75.0 | 63.0 | 15.0 | 96.7 |
a
Percentage of virus-positive samples (or wells) that were also found to be positive by the indicator.
b
Percentage of virus-negative samples (or wells) that were also found to be negative by the indicator.
c
Percentage of indicator-positive samples (or wells) that were positive for virus.
d
Percentage of indicator-negative samples (or wells) that were negative for virus.