Convert Probability to Logit

logit() converts a probability to the corresponding logit value.

prob_from_logit() converts a logit value to the corresponding probability.

Usage

logit(x)

prob_from_logit(x)

Arguments

x

a probability from which to derive the logit, or a logit from which to derive the probability.

Value

logit returns the derived logit; prob_from_logit returns the derived probability.

Details

The logit is defined as the log of the odds or \(\log(p / q)\), where \(q = 1 - p\).

The probability may be calculated from the logit by the inverse transformation: -

$$p = \exp(\displaystyle \frac{logit}{1 + logit})$$

See also

exp, log, plogis and qlogis and the Note section of Logistic in the stats package.

Other odds-logit: odds()

Examples

logit(0.5)
#> [1] 0
prob_from_logit(0)
#> [1] 0.5

(s <- seq(-10, 10, by = 2))
#>  [1] -10  -8  -6  -4  -2   0   2   4   6   8  10
s[1:6] + s[11:6]
#> [1] 0 0 0 0 0 0

(s <- prob_from_logit(s))
#>  [1] 4.539787e-05 3.353501e-04 2.472623e-03 1.798621e-02 1.192029e-01
#>  [6] 5.000000e-01 8.807971e-01 9.820138e-01 9.975274e-01 9.996646e-01
#> [11] 9.999546e-01
s[1:6] + s[11:6]
#> [1] 1 1 1 1 1 1

(s <- logit(s))
#>  [1] -10  -8  -6  -4  -2   0   2   4   6   8  10

rm(s)