PHP Unconference Europe 2015

stats_standard_deviation

(PECL stats >= 1.0.0)

stats_standard_deviationRetourne l'écart type

Description

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

Liste de paramètres

a

Le tableau de données sur lequel l'écart type doit être calculé. Remarque : toutes les valeurs du tableau seront converties en float.

sample

Indique si a est un échantillon. Par défaut : FALSE.

Valeurs de retour

Retourne l'écart type en cas de succès, FALSE si une erreur survient.

Erreurs / Exceptions

Une erreur E_WARNING est émise s'il y a moins de 2 valeurs dans a.

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User Contributed Notes 5 notes

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2
levim at php dot net
8 months 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);
    }
}

?>
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1
Pascal Woerde
4 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);
}
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1
Sharon
4 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) );
}

?>
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0
tripollite at gmail dot com
2 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) ;
    }
}
?>
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-1
pmcdougl at gac dot edu
2 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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