A fine description of the difficulties of software estimation:

1. It’s important to remember that the relationship between points and hours is not an equivalence. It is not that 1 point equals 4 hours (which is what our example showed above). It is that 1 point equals a mean of four hours with a standard deviation of plus or minus say 45 minutes. This would mean that most of the time (68% of the time) the relationship would be that 1 point takes from 3:15 to 4:45 to finish. It would mean that almost always (98% of the time), one point would take between 2-1/2 and 5-1/2 hours to finish (two standard deviations). 2. The above approach assumes that 1-point stories and 13-point stories are estimated perfectly in relative terms. In other words, it assumes that if the mean duration of a one-point story is 4 hours, the mean of a 13-point story will be 13x4=52 hours. For many reasons, this is unlikely to be true. And the data I’ve collected from a variety of teams, shows that–as we’d expect–teams are not perfect, even though many are amazingly consistent.