public function computeDelta()
    {
        if (!$this->hasDelta()) {
            return false;
        }

        // computes this and the most recently completed period
        $thisPeriod = $this->nthPreviousPeriod();
        $mostRecentPeriod = $this->nthPreviousPeriod(1);

        $current = $this->computeValue($thisPeriod);
        $previous = $this->computeValue($mostRecentPeriod);

        return $current - $previous;
    }