Very insightful comment, thank you for posting it. 
Let me put a positive spin on it. While the unconditional variance is so large that the statement is meaningless, if you condition on your 3 factors, I think you can obtain a pretty good predictor.<p>For example, for the combination 1. activity = running, 2. person = world champion in cycling, 3. competence = sub-3-hour marathon, then you would expect the time to be around 100 hours. Lance Armstrong fits this combination. How long he trained, we can&#x27;t know for sure, but the first google hit for (a free) marathon training program at the elite level [1] shows 85h total running time.<p>[1] <a href="http:&#x2F;&#x2F;www.yourmarathontrainingplan.com&#x2F;free-training-programs&#x2F;elite&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.yourmarathontrainingplan.com&#x2F;free-training-progra...</a>