Also, how useful is measuring the percentage of solid ground compared to last fall? What if the wind is less, the summer later, or the leave growth from the previous summer less? Also, other factors besides leaves could make the ground problematic for children, such as if it were flooded.

This comparison works for me except testing is also estimating how long it will take to rake leaves in the future without a weather report or a count on the number of children on swings to push.