// normalize contact exposure rate by number of people in age groups
        for (auto age_group = AgeGroup(0); age_group < AgeGroup(num_agegroups); age_group++) {
            for (auto& v : cell.m_cached_exposure_rate_contacts.slice(AgeGroup(age_group))) {
                auto number_persons_in_age_group =
                    std::count_if(cell.m_persons.begin(), cell.m_persons.end(), [age_group](observer_ptr<Person> p) {
                        return p->get_age() == age_group;
                    });
                if (number_persons_in_age_group > 0) {
                    v = v / number_persons_in_age_group;
                }
            }
        }