stats_stat_percentile

(PECL stats >= 1.0.0)

stats_stat_percentile — Returns the percentile value

说明

float stats_stat_percentile ( array $arr , float $perc )

Returns the perc-th percentile value of the array arr.

参数

arr

The input array

perc

The percentile

返回值

Returns the percentile values of the input array.

User Contributed Notes

aboulang2002 at yahoo dot com 29-Jan-2009 02:22
If you are looking to infer the percentile from a z-score, you can use this function.
It's far from precise but does the job on most circumstances.
The error function ( erf() )is based on the approximation on wikipedia:
http://en.wikipedia.org/wiki/Error_function

<?php
function erf($x)
{
        $pi = 3.1415927;
        $a = (8*($pi - 3))/(3*$pi*(4 - $pi));
        $x2 = $x * $x;

        $ax2 = $a * $x2;
        $num = (4/$pi) + $ax2;
        $denom = 1 + $ax2;

        $inner = (-$x2)*$num/$denom;
        $erf2 = 1 - exp($inner);

        return sqrt($erf2);
}

function cdf($n)
{
        if($n < 0)
        {
                return (1 - erf($n / sqrt(2)))/2;
        }
        else
        {
                return (1 + erf($n / sqrt(2)))/2;
        }
}

// EXAMPLE
$sample = 90;
$avg = 75;
$stddev = 12;

$zscore = ($sample - $avg) / $stddev;
print 'Percentile: ' . cdf($zscore) * 100 . "\n";
?>

Where $n is the z-score

The function cdf() returns the approximed cumulative standard normal distribution ([0..1])



[EDIT BY danbrown AT php DOT net: Contains a bugfix provided by (Ed) on 24-FEB-2010 which fixes the definition of $a in erf() with the note that it is "out by a factor of -1" in the original code.]
yuvaraj dot v at gmail dot com 11-Dec-2007 01:37
I have looked at the code available in the Math package for percentile. I compared the result obtained with percentile from Excel and found that it doesnt match. So i wrote my own percentile function and verified the result with the Excel's percentile.

For those of you who need the percentile calculation of Excel in php..

<?php
function mypercentile($data,$percentile){
    if( 0 < $percentile && $percentile < 1 ) {
        $p = $percentile;
    }else if( 1 < $percentile && $percentile <= 100 ) {
        $p = $percentile * .01;
    }else {
        return "";
    }
    $count = count($data);
    $allindex = ($count-1)*$p;
    $intvalindex = intval($allindex);
    $floatval = $allindex - $intvalindex;
    sort($data);
    if(!is_float($floatval)){
        $result = $data[$intvalindex];
    }else {
        if($count > $intvalindex+1)
            $result = $floatval*($data[$intvalindex+1] - $data[$intvalindex]) + $data[$intvalindex];
        else
            $result = $data[$intvalindex];
    }
    return $result;
}
?>

The above code may not be elegant.. but it solves my problem..

yuvaraj