R/lnorms.R

In popbio: Construction and Analysis of Matrix Population Models

#' Generate random lognormal values for fertility rates
#'
#' Converts standard normal random values to lognormals with defined means and variances
#'
#' converted Matlab code from  Box 8.4 in Morris and Doak (2002)
#'
#' @param n number of observations
#' @param mean mean value of the fertility rate, default 2
#' @param var variance of the vital rate (not standard deviation), default 1
#'
#' @return A vector of random lognormal values
#'
#' @note This function could probably be replaced with built-in functions
#' for the Log Normal Distribution \code{\link{rlnorm} }
#'
#' @references Morris, W. F., and D. F. Doak. 2002. Quantitative conservation
#' biology: Theory and practice of population viability analysis. Sinauer,
#' Sunderland, Massachusetts, USA.
#'
#' @seealso \code{\link{stretchbetaval}}
#'
#' @author Original Matlab code by Morris and Doak (2002: 281). Adapted to R by
#' Patrick Nantel, 20 June 2005.
#'
#' @examples
#' lnorms(1)
#' # Generate lognormal random fertilities
#' # for a population of 1000 mature individuals with mean fertility of
#' # 3 and inter-individual variance in fertility of 1.5.
#' rndfert  <- lnorms(1000, 3,1.5)
#' summary(rndfert)
#' hist(rndfert,40, main="Lognormal random fertilities",
#' xlab="Fertility rate", col="blue")
#'
#' @export

lnorms <- function(n, mean = 2, var = 1) {
  nmeans <- log(mean) - 0.5 * log(var / mean^2 + 1)
  nvars <- log(var / mean^2 + 1)
  normals <- rnorm(n) * sqrt(nvars) + nmeans
  lns <- exp(normals)
  lns
}