public function initialise_for_slot($slot, $question, $variant = null) {
        $newqstat = new calculated($question, $slot, $variant);
        if ($variant === null) {
            $this->questionstats[$slot] = $newqstat;
        } else {
            $this->questionstats[$slot]->variantstats[$variant] = $newqstat;
        }
    }