stats_stat_percentile

(PECL stats >= 1.0.0)

stats_stat_percentile — Returns the percentile value

说明

stats_stat_percentile(array $arr, float $perc): float

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.

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aboulang2002 at yahoo dot com ¶

15 years ago

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.]

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yuvaraj dot v at gmail dot com ¶

16 years ago

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