Two group discriminant diagnostics

For 2 x 2 tables there are four terms used to summarise the classification table of observed and predicted group membership outputted by discriminant procedures such as binary logistic regression.

Let’s call the two groups positive (+) and negative (-) with classification table given below.

The following four quantities are often quoted and asked for by journals as a means of evaluating the sharpness of the model fit in the two group case.

• Sensitivity = a/(a+c)= proportion of those who are really + who are predicted to be +
• Specificity = d/(b+d) = proportion of those who are really - who are predicted to be -
• Positive Predictive Value (PPV) = a/(a+b) = proportion of those predicted as + who really are +
• Negative Predictive Value (NPV) = d/(c+d) = proportion of those predicted as - who really are -

[:FAQ/wavelength_eg An example] using wavelength data.

References

Fleiss, JL (2003) Statistical Methods for Rates and Proportions, 3rd Edition. Wiley:New York. (in South Wing 5 of University library).