Using data that’s already collected—and that is self-reported (no one’s memory is 100 percent accurate)—can make it hard to draw firm conclusions, as the study authors note. It’s possible too that going outside doesn’t make you feel healthier, but instead, healthier people simply have more time and energy to make it to a park. However, the study authors say they still have reason to think that’s not the case here. A weekly two hours of outdoor time was equally beneficial to elderly and disabled people whose physical conditions could make it harder to get outside. The benefit to all people, not just the healthy, suggests the finding “isn’t simply due to healthier people visiting nature more often,” the authors wrote.