stats_standard_deviation

    (PECL stats >= 1.0.0)

    stats_standard_deviation — Returns the standard deviation

    Description

    stats_standard_deviation(array $a, bool $sample = false): float

    Returns the standard deviation of the values in a.

    Parameters

    a

    The array of data to find the standard deviation for. Note that all values of the array will be cast to float.

    sample

    Indicates if a represents a sample of the population; defaults to false.

    Return Values

    Returns the standard deviation on success; false on failure.

    Errors/Exceptions

    Raises an E_WARNING when there are fewer than 2 values in a.

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    User Contributed Notes 5 notes

    up
    16
    levim at php dot net
    10 years ago
    Here's an implementation that adheres to the PECL source quite strictly. This is useful for people who want to use it but don't have the extension, as well as for people who want to understand what is going on.

    <?php

    if (!function_exists('stats_standard_deviation')) {
    /**
    * This user-land implementation follows the implementation quite strictly;
    * it does not attempt to improve the code or algorithm in any way. It will
    * raise a warning if you have fewer than 2 values in your array, just like
    * the extension does (although as an E_USER_WARNING, not E_WARNING).
    *
    * @param array $a
    * @param bool $sample [optional] Defaults to false
    * @return float|bool The standard deviation or false on error.
    */
    function stats_standard_deviation(array $a, $sample = false) {
    $n = count($a);
    if (    $n === 0) {
    trigger_error("The array has zero elements", E_USER_WARNING);
    return     false;
    }
    if (    $sample && $n === 1) {
    trigger_error("The array has only 1 element", E_USER_WARNING);
    return     false;
    }
    $mean = array_sum($a) / $n;
    $carry = 0.0;
    foreach (    $a as $val) {
    $d = ((double) $val) - $mean;
    $carry += $d * $d;
    };
    if (    $sample) {
    --    $n;
    }
    return     sqrt($carry / $n);
    }
    }

    ?>
    up
    4
    Pascal Woerde
    13 years ago
    If you want one function for the population and sample, you can use this function:

    <?php

    function standard_deviation($aValues, $bSample = false)
    {
    $fMean = array_sum($aValues) / count($aValues);
    $fVariance = 0.0;
    foreach (    $aValues as $i)
    {
    $fVariance += pow($i - $fMean, 2);
    }
    $fVariance /= ( $bSample ? count($aValues) - 1 : count($aValues) );
    return (float)     sqrt($fVariance);
    }
    up
    3
    Sharon
    14 years ago
    If you don't have the stat package you can use these functions:

    <?php

    // Function to calculate square of value - mean
    function sd_square($x, $mean) { return pow($x - $mean,2); }

    // Function to calculate standard deviation (uses sd_square)
    function sd($array) {

    // square root of sum of squares devided by N-1
    return sqrt(array_sum(array_map("sd_square", $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)-1) );
    }

    ?>
    up
    -2
    tripollite at gmail dot com
    12 years ago
    Function to calculate square of value - mean

    <?php
    function sd_square($x, $mean) { return pow($x - $mean,2); }
    ?>

    Function to calculate standard deviation (uses sd_square)
    <?php
    function sd($array) {

    // square root of sum of squares devided by N-1
    return sqrt(array_sum(array_map("sd_square", $array, array_fill(0,count($array), (array_sum($array) / count($array)) ) ) ) / (count($array)) );
    }
    ?>

    works better if you don't add +1 to the count

    standard deviation of a constant must be 0 ( http://en.wikipedia.org/wiki/Standard_deviation )

    different way :
    <?php
    class Tmath{
    public static function     variance($data, $round = 2)
    {
    if(     count($data) == 0 )
    return     NULL;
    $total_ecart = 0;
    $moyenne = self::moyenne($data, 10*$round);
    if(     $moyenne == 0)
    return     0;
    foreach(     $data as $element)
    {
    $total_ecart += pow($element, 2);
    }
    return     round($total_ecart/count($data) - pow($moyenne,2),$round);
    }
    public static function     ecartType($data, $round = 2)
    {
    return     round(sqrt(self::variance($data,10*$round)),2);
    }
    public static function     moyenne($data, $round = 2)
    {
    if(     count($data) == 0 )
    return     NULL;
    return     round(array_sum($data)/count($data),$round) ;
    }
    }
    ?>
    up
    -5
    pmcdougl at gac dot edu
    12 years ago
    It should be known that this function also casts all of the values of the array to floats. If you pass an array of integers, they will come out as an array of floats.

    $data = array(2,3,3,3,3,2,5);
    var_dump($data);
    $standardDeviation = stats_standard_deviation($data);
    var_dump($data);

    Prints:
    array
    0 => int 2
    1 => int 3
    2 => int 3
    3 => int 3
    4 => int 3
    5 => int 2
    6 => int 5
    array
    0 => float 2
    1 => float 3
    2 => float 3
    3 => float 3
    4 => float 3
    5 => float 5
    6 => float 2
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