This problem is VERY real and happens in surprising ways. My biotech has a dataset that was collected over several years and we found this pattern of older peop

This problem is VERY real and happens in surprising ways. My biotech has a dataset that was collected over several years and we found this pattern of older people having more Vitamin D in their blood than younger people, which is the opposite of the published literature. It turns out that the sample collection was initially year round but switched to summer only and everyone in the cohort was older, by definition, in the later collection.