Table 1.

The effect of prevalence on predictive summary index and number needed to predict when using an excellent sign, symptom or laboratory test*

Prevalence (Pre-test probability)	1%	10%	50%	90%	99%
Positive predictive value (PPV)	16%	68%	95%	99.4%	99.9%
Negative predictive value (NPV)	99.9%	99.4%	95%	68%	16%
Predictive summary index (PSI) (NPV + PPV − 1)	15.9%	67.4%	90%	67.4%	15.9%
Number needed to predict (NNP) (1/PSI)	6.3	1.4	1.1	1.4	6.3

Sensitivity and specificity equal 95% in every case.

Table constructed using data presented by Sackett et al.⁶