Prevalence Scaling

prevalence.scaling <- function(model.P, model.prevalence = 0.5, target.prevalence = 0.1)
{
    ## Purpose: Convert a probability based on training data prevalence to
    ##          a probability based on target population prevalence. 
    ## Arguments:
    ##   model.P: a vector of model-based probability (risk score)
    ##   model.prevalence: prevalence of the model's training data
    ##   target.prevalence: prevalence of the target population
    ## Return:
    ##   a vector of scaled probabilities reflecting the prevalence of the target population
    ## Author: Feiming Chen
    ## ________________________________________________

    ## convert a probability to the odds
    O <- function(p)  p / (1-p)

    1 / ( 1 + O(model.prevalence) / O(target.prevalence) / O(model.P) )
}
if (F) {                                # Unit Test
    set.seed(1)
    prevalence.scaling(runif(5))
    ## [1] 0.038614 0.061784 0.129688 0.523663 0.027304
}