Variation in Poison Ivy

You know how hard it can be to compare some random plant to the picture you sort of memorized from a book?  It looks close, maybe, but it's still quite different than the reference sketch in some field guide?  Well surprise, the appearance of poison ivy is all over the place.  Article at NPR.

Those four examples hardly look like the same kind of plant.  This has all kinds of implications for outdoor activities.