[bckygldstn]

The title of the NPR article would be more accurate as "Cities See a Reduction in Ambulance Usage After Uber is Introduced".

The paper looks at 700 different cities, each with a different Uber market entry time over a two year period. The researchers also control for some seasonality effects.

Perhaps more covariates could have been considered, but this seems like a pretty decent design to measure a before/after causal effect.