sub std_dev {
  my ($sum, $sqr_sum);
  for (@_) {
    $sum += $_;
    $sqr_sum += $_ * $_;
  }
  sqrt(($sqr_sum - $sum * $sum / @_)/@_);
}

##</code><code>##

# Produces an unbiased estimate of a standard deviation
# from a sample
sub std_dev_samp {
  my ($sum, $sqr_sum);
  for (@_) {
    $sum += $_;
    $sqr_sum += $_ * $_;
  }
  sqrt(($sqr_sum - $sum * $sum / @_)/$#_);
}