I’m a little bit afraid of asking a stupid question here, I’m confused about sizing an MMR as described in Chapter 20 Improving While in the Flow of TameFlow. Possibly because I don’t grasp Little’s Law as I should.
I’m running a small experiment at the moment, trying to determine an MMR of a project consisting of 15 Product Backlog Items (PBIs), same class of service (roughly same size). Looking at the history of a past project I was able to determine that the average Flow Time of our PBIs is 15 work days (excluding weekends). By Flow Time I mean time work started by a developer to time the PBI was Done.
Then I divided the 15 days by two and rounded to 7 days as buffer.
If I aggregate these numbers I get:
- MMR Flow Time = 15 PBIs X 15 days = 225 days
- MMR Buffer = 15 PBIs X 7 days = 105 days
My confusion lies on Little’s Law -> Flow time = WIP / Throughput.
225 days to complete 15 PBIs? When I look back at the previous project (coincidentally it was 15 PBIs) work on the first PBI started on 12/02/2019 and the last one was finished on 28/06/2019 (that is Throughput = 15 PBIs / 99 work days = 0.1515).
So previous project Ft = 15 / 0.1515 = 99 days.
I was expecting to see the MMR Flow Time to be similar to our past performance, but what I get is 225 days vs 99 days. This looks like a big discrepancy to me.
Are my calculations correct?
What Am I missing?