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);

Answer 2

The answer above could throw an undefined index notice if you use the higher percent value (100) and does not return correct values according to Excel PERCENTILE function. You can see here an example of how it fails.

I've written a function in PHP according the Wikipedia Second varitant, which is the one used in Excel. This function is also protected from a non percentual value (out of range).

function getPercentile($array, $percentile)
{
    $percentile = min(100, max(0, $percentile));
    $array = array_values($array);
    sort($array);
    $index = ($percentile / 100) * (count($array) - 1);
    $fractionPart = $index - floor($index);
    $intPart = floor($index);

    $percentile = $array[$intPart];
    $percentile += ($fractionPart > 0) ? $fractionPart * ($array[$intPart + 1] - $array[$intPart]) : 0;

    return $percentile;
}