use List::Util 'sum';

sub sample_sd {
  die unless @_ and ref $_[ 0 ] eq 'ARRAY' and @{ $_[ 0 ] } > 1;
  my $data = shift;
  my $sum = sum @$data;
  my $sum2 = sum map $_*$_, @$data;
  my $n = @$data;
  return sqrt( ( $sum2 - (( $sum*$sum )/$n) )/($n-1) );
}