I've always had a problem with the scrum concept of team velocity i.e. assigning story points to tasks and then measuring how many story points a team delivers each sprint. The idea that this yeilds any predictive power is flawed. It's based on the assumption that the team is static. As soon as someone leaves the team or someone new joins, all the historical velocity data becomes useless and you need to recalibrate. The reality is that teams are dynamic. They grow/shrink/change all the time.