[1]

sub get_total_of (@) {
  my $sum = 0;
  for (@_) {
    $sum += $_;
  }
  return $sum;
}

sub get_mean_of (@) {
  return get_total_of (@_) / (scalar @_ or 1);
}

sub get_variance_of (@) {
  my $mean = get_mean_of (@_);
  return get_mean_of (map { ($_ - $mean) ** 2 } @_);
}

sub get_confidence_interval_of ($@) {
  my $percentage = shift;
  my $z = {
    90 => 1.645, 95 => 1.960, 99 => 2.576, 99.9 => 3.29,
  }->{$percentage};
  return undef unless defined $z;
  my $stddev = sqrt (get_variance_of (@_));
  my $n = @_ || 1;
  return $z * $stddev / sqrt $n;
} # get_confidence_interval_of