R/deltadist.R

In fishmethods: Fishery Science Methods and Models

deltadist<-function (x = NULL) 
{
  if (is.null(x)) 
    stop("catch data are missing.")
  if (!is.numeric(x)) 
    stop("catch data are not numeric.")
  G <- function(t) {
    j <- 2
    s <- (((m - 1)^3)/((m + 1) * (m^2) * 2)) * t^2
    a <- 1 + (((m - 1)/m) * t) + s
    while (s > 1e-06) {
      j <- j + 1
      b <- (((m - 1)^2)/(m * (m + 2 * j - 3) * j)) * t
      s <- s * b
      a <- a + s
    }
    a
  }
  x <- x[!is.na(x)]
  n <- length(x)
  m <- length(x[x > 0])
  if (m > 1) {
    my <- mean(log(x[x > 0]))
    vary <- var(log(x[x > 0]))
    c <- (m/n) * exp(my) * G(0.5 * vary)
    varc <- (m/n) * exp(2 * my) * ((m/n) * (G(0.5 * vary)^2) - 
                                     (((m - 1)/(n - 1)) * G(((m - 2)/(m - 1)) * vary)))
  }
  if (m == 1) {
    c <- x[x > 0]/n
    varc <- (x[x > 0]/n)^2
  }
  if (m == 0) {
    c <- 0
    varc <- 0
  }
  outpt <- unlist(list(deltamean = c, var.mean = varc))
  return(outpt)
}