"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 Vaporfl

"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