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    stats_stat_percentile

    (PECL stats >= 1.0.0)

    stats_stat_percentile — Returns the percentile value

    Description

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

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

    Parameters

    arr

    The input array

    perc

    The percentile

    Return Values

    Returns the percentile values of the input array.

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    User Contributed Notes 2 notes

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    8
    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.]
    up
    4
    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
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