[__initbrian__]

"A 2018 New York Times data analysis based on public race results uploaded to Strava, the athlete-tracking and networking company, found that runners in Vaporflys ran 3 to 4 percent faster than similar runners wearing other shoes." This doesn't seem causal... Runners that were on average going to be 4 percent faster than the population mean anyway may also be runners that like buying the latest and greatest vaporflys. With that being said, I would have trouble designing an experiment though

[Doxin]

Get a bunch of runners, hand them random shoes. Have em run and time it. Repeat over the course of a couple days, having a randomised distribution of shoes each day. For bonus points use covers on the shoes so people don't know which shoes they are wearing.