I don't know how people could estimate User Stories' effort in any time-independent units. I read that someone measure effort in "green frogs" or "pink bunnies", but I really don't understand how they could completely abstract they mind from time units. Days, hours or weeks are so natural that I wonder why I should use abstract (well, but funny) units.
I stick to ideal day. Load factor is just bad in many cases and does not bring anything interesting. Ideal days are good and easy to understand. I can imagine and estimate how much work I could accomplish within a single ideal day. No interruption, no coffee breaks, great mood and so on. So I estimate User Stories in ideal days. I believe that precision is much better in this case.
However, there are some problems. The major one is that my ideal day is not the same as others' ideal days. For example, I may complete the "Tricky business rule implementation" story within 4 ideal days, Bob will complete it within 3 ideal days, and our super-developer Joe will get it done within 1 day. So my estimates will be completely invalid if User Story will be assigned on Joe.
Do you think I must stop estimating? Maybe I should change the rules, try to use "average developer's performance", invite cool consultant and start to count Functional Points? Right? Wrong! In reality I DON'T CARE about individual performance difference. I won’t emphasize that Bob three times slower than Joe. I won’t break the team. I just measure team velocity (or iteration velocity), since Joe will always do about the same amount of work, and Bob will do about the same (but different) amount of work as well during one iteration. The main idea is that effort units and assumptions for estimation must remain the same during the whole project. So if I chose My Ideal Day as the velocity unit, I must estimate all user stories on my ideal day basis. Always.
That’s all. Yes, estimation in agile project may be that simple 🙂 And to measure effort in "green frogs" I should just say that my ideal day equals to one green frog.