shannon.proc <- function(pvect){
  #
  #  Procedure to compute the normalized Shannon heterogeneity measure
  #
  #  Note: the input variable pvect is assumed to be a normalized 
  #        probability vector that sums to 1 and is of length M.
  #        Missing values are not allowed, but components of pvector
  #        are allowed to be zero.
  #
  M <- length(pvect)
  if (M == 1){
    IShannon <- 0
  }
  else{
    normfactor <- log(M)
    rawvect <- pvect * log(pvect)
    #
    #  Note: zero components of pvect cause log(pvect) to
    #        evaluate to NaN; here, adopt the convention
    #        that p log (p) = 0 when p = 0 and replace NaN's
    #
    rawvect[which(is.na(rawvect))] <- 0
    #
    rawsum <- sum(rawvect)
    IShannon <- 1 + rawsum/normfactor
    #
  }
  round(IShannon,digits=6)
  #
}