# a function to return a vector of residuals, with NAs where they belong.
residuals.with.na <- function(x,y){
  resids <- lm(y~x)$resid
  missing <- is.na(x+y)
  data.out <- rep(NA,length(missing))
  cdi <- 1   # cdi=current data index
  for (i in 1:length(missing)){
    if(!missing[i]){
      data.out[i] <- resids[cdi]
      cdi <- cdi+1
    }
  }
  return(data.out)
}