Maybe it is a self-filtering effect, they should have shown a graph of shoe price vs improvement, and improvement over time. That would capture if those willin

Maybe it is a self-filtering effect, they should have shown a graph of shoe price vs improvement, and improvement over time. That would capture if those willing to pay more are self-selecting for being better, training more, etc. And if today the effect is stronger than a year ago, perhaps because of rumors of it making you faster.