protected function _calculateLetterScore()
    {
        $score = 0;

        foreach (array (self::CLASS_UPPER, self::CLASS_LOWER) as $class) {
            $letterCount = $this->_getClassCount($class);

            if ($letterCount != $this->_length) {
                if ($letterCount > 0) {
                    $score += ($this->_length - $letterCount) * 2;
                }
            }
        }

        return $score;
    }