Boustan and her colleagues needed to set up their data as if it was an experiment — two groups, one gets a treatment and the other one doesn’t. They did that by using the fact that some cities enacted rental protections and some didn’t, and compared the average level of gentrification in each type of city. If rental protections are effective, there would be fewer condo conversions in the core of cities that enacted them. If living in a condo is a high driver of demand for rich people, then in protectionist cities, those people would be less likely to move to the core (because of fewer condos) and more likely to move to the suburbs — and the city’s core would experience less gentrification.