I saw this old Stephen Dubner article from the New York Times Magazine pop up on Longform this week and thought it was a great story.
A former economist decides to quit his job and start selling bagels on the honor system at a few hundred offices in the Washington, DC area. He leaves a couple dozen bagels and a money box, then comes back at the end of the day to collect.
In the process he learns quite a bit about what kinds of offices are more honest in paying for the bagels than others and what external factors affect payment (weather, holidays, etc.) He collected so much data he can now fairly reliably predict what the payment rate will be for any kind of office he sells his bagels at.