One of my students in environmental philosophy (Greg Puleo, a mathematician-to-be) constructed an Internet footprinting quiz worth visiting. The Guilt-o-Vision addresses the problem with other quizzes' hidden and baroque architecture by simplifying land use down to the numbers for crops and livestock.
While other quizzes measure a bounty of factors and crunch them down to one opaque figure (such as this one, which says that I would need 3.2 earths if everyone were to drive, eat, heat their house, pay Terrapass, and throw out their garbage the way I do), Greg's quiz just tells you how many square kilometers of land are required to grow your food. Well, not all your food, but corn, chicken, and beef.
What's astonishing about what it shows, of course, is that if you substitute corn for meat, much less land is used. However, Greg has pointed out that the variables and how they're measured incorporate many assumptions. For instance, there is a difference between corn and other vegetables, between industrial and non-industrial production methods, and between factory-raised chickens and farm-raised chickens. Not to mention that in an efficient agriculture system, animals can be raised on pasture which is too poor for growing crops. But that's not how our animals are raised, are they? They're raised on mid-Western corn.