Table 6.

Extent of bias associated with misclassification* of the effectiveness algorithm according to observed response rate

Observed response rate	True response rate**	Bias (observed-true)/true (%)
30%	30%	< 1%
40%	36%	10%
50%	43%	16%
60%	49%	21%

*From Table 4, where the positive predictive value (PPV) was 76% and the negative predictive value (NPV) was 90%. **Computed as True rate = Observed rate × (PPV + NPV-1)-NPV + 1 [39]. Numbers are rounded to the nearest whole integer but the actual values were used to calculate the bias.