How can I calculate the nth percentile from an array of doubles in PHP?

Question

I have a large array of doubles and I need to calculate the 75th and 90th percentile values for the array. What\'s the most efficient way to do this via a function?

Answer 1

It's been awhile since statistics, so I could be off here - but here's a crack at it.

function get_percentile($percentile, $array) {
    sort($array);
    $index = ($percentile/100) * count($array);
    if (floor($index) == $index) {
         $result = ($array[$index-1] + $array[$index])/2;
    }
    else {
        $result = $array[floor($index)];
    }
    return $result;
}

$scores = array(22.3, 32.4, 12.1, 54.6, 76.8, 87.3, 54.6, 45.5, 87.9);

echo get_percentile(75, $scores);
echo get_percentile(90, $scores);