The largest study of its kind — analyzing data from 24,763,389 results between 1996 to 2016 — has found that the average American runner, from 5k runners to marathoners, is getting slower.

The study was led by Jens Jakob Andersen and Ivanka Andreeva Nikolova from Andersen is a former competitive runner and statistician from Copenhagen Business School. Nikolova holds a Ph.D. in Mathematical Analysis. The results include only races with more than 2,000 finishers. The researchers have data from 1902 to 2017 (May), but chose to publish only the trimmed data set from 1996 to 2016. Results can be trusted at a 99 percent confidence level (a statistical measure based on the sample size) and don’t include top elite American runners, who are faster than ever.

Source: Why Are American Runners Getting Slower? : 13.7: Cosmos And Culture : NPR