Sarah Kunst knew the elements of a successful startup from her tenure at venture capital firms. In April 2018, however, her own app - Proday, a home fitness platform featuring exercises filmed by professional sports stars - was floundering. Kunst theorized that Facebook algorithm changes had ruined her marketing plan, but needed to be confident in her diagnosis before allocating resources to a solution. She thought back to Proday's launch two years earlier to see if she could pinpoint where it all went wrong.