public function getDuration()
    {
        if ($this->isRoot() && $this->profiles) {
            // for the root node with children, duration is the sum of all child durations
            $duration = 0;
            foreach ($this->profiles as $profile) {
                $duration += $profile->getDuration();
            }

            return $duration;
        }

        return isset($this->ends['wt']) && isset($this->starts['wt']) ? $this->ends['wt'] - $this->starts['wt'] : 0;
    }