Originally Posted by
surak
It's arbitrary because without a semblance of a protocol, Garmin misses many confounding factors like the fact that I wasn't riding to exhaustion. Those 20 minutes were during a climb that happened to take me about that long, but with 2 more hours of riding ahead. Another indication of its arbitrariness: none of my outdoor rides are what I would consider useful for estimating FTP, and Garmin only ever managed to find that one ride to produce an estimate.
It picks the best 20 minute block of time you have done. It doesn’t know if that was assisted or not or other factors. It is a good place to start, however. As I said previously, a rider should find which type of test gives them their best numbers stick with it for most consistent results.