Logit Transformation

Compute the logit transformation of proportions or percentages.

Usage

logit(p, percents, adjust)

Arguments

p

numeric vector or array of proportions or percentages.

percents

TRUE for percentages, FALSE for proportions. If the argument is missing and the largest value of p > 1, percents is set to TRUE, otherwise to FALSE.

adjust

adjustment factor to avoid proportions of 0 or 1; defaults to 0 if there are no such proportions in the data, and to .025 if there are.

Details

Computes the logit transformation logit \(=\log [p/(1-p)]\) for the proportion \(p\).

If \(p=0\) or \(1\), then the logit is undefined. logit can remap the proportions to the interval (adjust, 1 - adjust) prior to the transformation. If it adjusts the data automatically, logit will print a warning message.

Value

a numeric vector or array of the same shape and size as p.

References

Fox, J. and Weisberg, S. (2019) An R Companion to Applied Regression, Third Edition, Sage.

Author

John Fox jfox@mcmaster.ca and Iain Proctor

See also

probabilityAxis

Examples

save.opt <- options(digits=4)
logit(.1*0:10)
#> Warning: proportions remapped to (0.025, 0.975)
#>  [1] -3.6636 -1.9924 -1.2950 -0.8001 -0.3847  0.0000  0.3847  0.8001  1.2950
#> [10]  1.9924  3.6636
logit(.1*0:10, adjust=0)
#>  [1]    -Inf -2.1972 -1.3863 -0.8473 -0.4055  0.0000  0.4055  0.8473  1.3863
#> [10]  2.1972     Inf
logit(10*1:9, percents=TRUE)
#> [1] -2.1972 -1.3863 -0.8473 -0.4055  0.0000  0.4055  0.8473  1.3863  2.1972
options(save.opt)