A recent example of this is an experiment by Rob Jensen and Nolan Miller, where they look at the effect on consumption of changes in the price of rice. If you decrease the price of rice, will people consume more rice or less rice? In the real world, it’s very difficult to know that because whenever the price of rice decreases, that’s the result of a combination of supply and demand factors, and isolating variation in the price of rice as purely exogenous is essentially impossible.So you need an experiment to know, and in fact they found something very interesting when they did this experiment in one place in China where rice is a very important part of the food basket for the poor. And they found that when the price decreased, people ate less rice, not more rice, which means rice is a Giffen good .