scores = function(...,fcn='mean',minvalid=1,score=NULL,minval=0,maxval=0)
#
# scores() calculates scores according to the argument of fcn using variables
# given at ... . If the number of valid values of ... is less than minvalid
# the resulting score will be set to missing.
#
# If fcn='mean' arguments of score= can be used to request a transformation of
# the scores to z-scores, to scores centered at the overall mean, or to POMP
# (= percent of maximum possible) scores with 0 and 100 as minimum and maximum
# possible values (see: Cohen, P., Cohen, J., Aiken, L.S. & West, S.G. (1999).
# The problem of units and the circumstance for POMP. Multivariate Behavioral
# Research, 34, 315-346). The latter requires to specify the minimum and maxi-
# mum possible values of the items given at ... by using minval= and maxval=.
#
# fcn   : possible arguments are 'min', 'max', 'sum', 'sd', and 'mean'
#         (default: 'mean')
# score : score= can only be specified if fcn='mean'. Possible arguments (trans-
#         formations) are 'centered' or 'c' (mean scores centered at the overall
#         mean), 'z' (z-scores), or 'pomp' or 'p' (pomp scores). The latter
#         requires to specify the minimum and maximum possible value of ... by
#         using minval= and maxval=, as well.
#
# Examples: see "http://www2.jura.uni-hamburg.de/instkrim/kriminologie/Mitarbeiter/Enzmann/Software/test_sc.r"
{
  if (!(fcn %in% c('min','max','sd','sum','mean'))) {
    stop(paste("'",fcn,"' is no valid argument of fcn",sep=""))
  }
  if (length(score)>0) {
    if (fcn!='mean') {
      stop(paste("score='",score,"' is only valid if fcn='mean'",sep=""))
    }
    if (!(score %in% c('centered','c','z','p','pomp'))) {
      stop(paste("'",score,"' is no valid argument of score",sep=""))
    }
    if (((score=='pomp') | (score=='p')) & (minval==maxval)) {
      stop("minval and maxval options should allow minval to be less than maxval")
    }
  }
  if (minvalid < 1) {
    stop("value specified in minvalid may not by less than 1")
  }
  temp  = cbind(...)
  if (dim(temp)[2] < minvalid) {
    stop(paste("value specified in minvalid (= ",minvalid,") may not be greater than\nthe number of variables (= ",dim(temp)[2],")",sep=""))
  }
  valid = apply(!is.na(temp),1,sum)
  sc = suppressWarnings(apply(temp,1,fcn,na.rm=T))
  sc[valid < minvalid] = NA
  if (length(score)>0) {
    if ((score=='centered') | (score=='c')) {
      sc = sc-mean(sc,na.rm=T)
    }
    if (score=='z') {
      sc.sd = sd(sc,na.rm=T)
      if (sc.sd==0) {
        sc = NA
        stop("variance of scores = 0")
      }
      else {
        sc = (sc-mean(sc,na.rm=T))/sc.sd
      }
    }
    if ((score=='pomp') | (score=='p')) {
      sc = 100*(sc-minval)/(maxval-minval)
      if (min(temp,na.rm=T) < minval) {
        warning("Value(s) of variables less than minval!")
      }
      if (max(temp,na.rm=T) > maxval) {
        warning("Value(s) of variables greater than maxval!")
      }
    }
  }
  sc
}