This could be assessed by randomly collecting all the leaves found on different twigs. Several twigs would be needed to get a meaningful result. Almost certainly, some of our twigs would contain many healthy beech leaves, others may contain few or none. This variation makes it difficult for us to be certain about our results. It also causes problems when we wish to compare beech leaf size in different habitats.
Beech trees vary in age and condition.
Sampling may vary. In the case of beech leaves it may rain one day and be sunny the next. Chance plays a part, especially with small sample sizes. A twig may just happen to have only large leaves. To get this you add up all your results and divide by the number in the sample.