sub correlate {
  my ($p1,$p2,$v) = @_;
  return if $p1 eq $p2;
  my ($xy,$x2,$y2);
  for my $date (sort keys %data){
    my $x = $data{$date}{$p1}{$v} - $total{$p1}{$v}{'mean'} ;
    my $y = $data{$date}{$p2}{$v} - $total{$p2}{$v}{'mean'} ;
    $xy += $x * $y;
    $x2 += $x * $x;
    $y2 += $y * $y;   
  }
  my $cor = $xy / sqrt($x2 * $y2); 
  print "$p1 $p2 $v $cor\n";
}