When I did that in the past I did not gather the whole team and started breaking down the back log in order to figure it out. or actually I used to, but the process was long and painful for everybody. That is why I started ignoring the team and just gathered a few experienced people representing different skills and did what I called "pull a number out of your ass" estimations. These estimates turned out to be good enough for high level planning. And they took less than an hour to complete. Now I'm happy that somebody have formalized it a little and given it a more politically correct name; Blink Estimation.
Blink Estimation in a nutshell is:
- Gather some smart people in a room, discuss the problem for a while.
- Then write down one estimate each and reveal to each other at the same time.
- Decide on what number to use.