statistics_stardard_deviation

[smerny]

http://us3.php.net/manual/en/function.stats-standard-deviation.php

 

Warning

 

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

 

this doesn't work for me, "undefined function", is there something I can do? or another function to use? or do I just need to create my own to use?

[Alex]

Well after 10 minutes of trying to figure out how to use 'bar x' in the [tex] tag..

 

You can install the math extension, or just create your own standard_deviation function, it's not that hard.. Ex:

 

<?php
function standard_deviation($arr)
{
$bar_x = array_sum($arr) / count($arr);
foreach($arr as $x)
	$dev[] = pow($x - $bar_x, 2);
return sqrt(array_sum($dev) / count($arr));
}

echo standard_deviation(Array(78, 84, 86, 92, 95)); // 6

[smerny]

okay, thanks

[Alex]

Figured it out, this is what I was trying to post originally..

[tex]\sigma = \sqrt{\frac{\sum_{n = 1}^n (x - \overline{x})^2}{n}}[/tex]

[smerny]

that's for population standard deviation btw, the denominator is n-1 for sample standard deviation

[Alex]

You didn't say you were trying to find sample standard deviation.

[Mchl]

If you have this data in MySQL, you can use one of these

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_var-pop

[smerny]

You didn't say you were trying to find sample standard deviation.

 

xbar is also for a sample