public function process()
    {
        $operands = $this->getOperands();

        if ($operands->containsNull() === true) {
            return null;
        }

        if ($operands->exclusivelyMultipleOrOrdered() === false) {
            $msg = "The StatsOperator operator only accepts operands with a multiple or ordered cardinality.";
            throw new OperatorProcessingException($msg, $this, OperatorProcessingException::WRONG_CARDINALITY);
        }

        if ($operands->exclusivelyNumeric() === false) {
            $msg = "The StatsOperator operator only accepts operands with a multiple or ordered cardinality.";
            throw new OperatorProcessingException($msg, $this, OperatorProcessingException::WRONG_BASETYPE);
        }

        $qtiFuncName = Statistics::getNameByConstant($this->getExpression()->getName());
        $methodName = 'process' . ucfirst($qtiFuncName);

        return call_user_func_array(array($this, $methodName), array());
    }