Glyn, I did something similar, but with mine after the granular tasks are estimated, a random delay is added to each according to a pareto distribution. The more subtasks, the more certain that a few of them will be very much behind schedule.
I chose a pareto distribution because it had the minimal number of parameters to estimate and it had a fat tail. Also I had a maximum entropy justification. Say you use an exponential distribution, you’re assuming a constant chance for completion at any time that it’s incomplete. But other things equal, the more you get behind schedule the less likely that the chance for cmpletion at any time will stay constant. It should go down. If you estimated 3 hours to completion to start with, and it’s already been 6 hours, is it more likely that the correct estimate now is 3 hours, or something larger? And when it’s been 9 hours and still incomplete, should you predict 3 hours then? The more you’re already behind deadline, the more reasonable it is to suppose that you’ll get even farther behind.
Glyn, I did something similar, but with mine after the granular tasks are estimated, a random delay is added to each according to a pareto distribution. The more subtasks, the more certain that a few of them will be very much behind schedule.
I chose a pareto distribution because it had the minimal number of parameters to estimate and it had a fat tail. Also I had a maximum entropy justification. Say you use an exponential distribution, you’re assuming a constant chance for completion at any time that it’s incomplete. But other things equal, the more you get behind schedule the less likely that the chance for cmpletion at any time will stay constant. It should go down. If you estimated 3 hours to completion to start with, and it’s already been 6 hours, is it more likely that the correct estimate now is 3 hours, or something larger? And when it’s been 9 hours and still incomplete, should you predict 3 hours then? The more you’re already behind deadline, the more reasonable it is to suppose that you’ll get even farther behind.