php 标准差

/**

 * 得到数组的标准差

 * @param unknown type $avg

 * @param Array $list

 * @param Boolen $isSwatch

 * @return unknown type

 */

public static  function getVariance($avg, $list, $isSwatch  = FALSE) {

        $arrayCount = count($list);

        if($arrayCount == 1 && $isSwatch == TRUE){

                return FALSE;

        }elseif($arrayCount > 0 ){

                $total_var = 0;

                foreach ($list as $lv)

                        $total_var += pow(($lv - $avg), 2);

                if($arrayCount == 1 && $isSwatch == TRUE)

                        return FALSE;

                return $isSwatch?sqrt($total_var / (count($list) - 1 )):sqrt($total_var / count($list));

        }

        else

                return FALSE;

}