PHP 7.0.6 Released

    stats_stat_percentile

    (PECL stats >= 1.0.0)

    stats_stat_percentile — Not documented

    Description

    float stats_stat_percentile ( float $df , float $xnonc )
    Warning

    This function is currently not documented; only its argument list is available.

    Parameters

    df

    xnonc

    Return Values

    User Contributed Notes

    yuvaraj dot v at gmail dot com
    8 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
    aboulang2002 at yahoo dot com
    7 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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