R/gen_binomial_anticoef.R

In skpr: Design of Experiments Suite: Generate and Evaluate Optimal Designs

#'@title Generates Binomial Anticipated Coefficients
#'
#'@description Generates Binomial Anticipated Coefficients
#'Solves the logistic function
#'log(p / (1-p)) = beta0 + beta1 * x
#'such that p = lowprob when x = -1, and p = highprob when x = +1.
#'Equivalently, solves this set of equations for beta0 and beta1:
#'log(lowprob / (1 - lowprob)) = beta0 - beta1
#'log(highprob / (1 - highprob)) = beta0 + beta1
#'@param anticoef Anticipated coefficients
#'@param lowprob Default 0.50. The base probability
#'@param highprob Default 0.80. The high probability
#'@return Anticipated coefficients.
#'@keywords internal
gen_binomial_anticoef = function(anticoef, lowprob, highprob) {
  if (highprob >= 1 || highprob <= 0 || lowprob >= 1 || lowprob <= 0) {
    stop("skpr: Binomial anticipated coefficients generation error: Probabilities must be between (and not equal to) 0 and 1")
  }
  b0 = 1 / 2 * log(lowprob * highprob / (1 - lowprob) / (1 - highprob))
  b = 1 / 2 * log(highprob * (1 - lowprob) / (1 - highprob) / lowprob)
  newanticoef = anticoef * b
  newanticoef[1] = b0
  return(newanticoef)
}