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 meaning

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. 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'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. [1] http://www.yourmarathontrainingplan.com/free-training-programs/elite/ http://www.yourmarathontrainingplan.com/free-training-progra...