public function setOutcomeIdentifier($outcomeIdentifier)
    {
        if (Format::isIdentifier($outcomeIdentifier, false) === true) {
            $this->outcomeIdentifier = $outcomeIdentifier;
        } else {
            $msg = "The 'outcomeIdentifier' argument must be a valid QTI identifier, '" . $outcomeIdentifier . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }