Adding 0.5 to cells when calculating odds ratios, d' and the empirical logit transform

On the use of adding 0.5 to cells to define an odds ratio

• Subbiah and Srinivasan (2008) note the practice of adding a constant to a zero cell to define an odds ratio. They go on to state "Usually a continuity correction, by addition of 0.5, is considered for studies with zero counts in the estimation of the log of the odds ratio or the log of the risk ratio."

• M Subbiah and M R Srinivasan (2008) Classification of 2x2 sparse data sets with zero cells. Statistics & Probability Letters 78(18) 3212-3215.

A pdf copy of this paper is available to read and print from here.

On the use of the +0.5 smoothing for calculating the empirical logit transform see:

• David R COX and E J SNELL (1989) Analysis of Binary Data , Chapman and Hall: London

On the use of the +0.5 smoothing for calculating the d' signal detection parameter see:

• Michael J HAUTUS (1995) Corrections for extreme proportions and their biasing effects on estimated values in d'. Behavioral Research Methods, Instruments, & Computers, 27(1), 46-51.

See also

Signal Detection Parameters from SPSS There a different convention is followed of putting 1/2N and 1-1/2N in place of 0% and 100%.

and here which suggests a nonparametric alternative to ROC curves which represents an average over all possible ROC curves.

These pages are maintained by Ian Nimmo-Smith and Peter Watson

Last updated on 27 July, 2006