protected function _variance($times, $sample = true)
{
$n = $mean = $M2 = 0;
foreach ($times as $time) {
$n += 1;
$delta = $time - $mean;
$mean = $mean + $delta / $n;
$M2 = $M2 + $delta * ($time - $mean);
}
if ($sample) {
$n -= 1;
}
return $M2 / $n;
}