### More Musings on Little's Law

My previous posting about why not  to use Cycle Time in Kanban resulted in some interesting discussions, and I'm grateful to +Steve Tendon for pointing me in the direction of this paper [1] by John D.C. Little and Stephen C. Graves which gives some very helpful historical background into the derivation of Little's Law, its applicability and some of the terminology used.

 From Little and Graves (2008)
Little's own formulation of the "law" was as follows:

L=Î»W

where

L = average number of items in the queuing system,
(equivalent to WIP in Kanban terminology)

W = average waiting time in the system for an item,

Î» = average number of items arriving per unit time
(equivalent to Delivery Rate, assuming "stationarity")

With Kanban preferred terms we can see this maps to:

WIP =  Delivery Rate * Lead Time
or
Delivery Rate = WIP / Lead Time

Little used "waiting time" for the time taken by one unit to traverse the system (W) because his original context was queuing systems. For other applications he suggested Flow Time, which I think is a very useful alternative.

He also notes though that other authors use other terms for W, including cycle time, throughput time, and sojourn time, depending on the context. Yes - cycle time I'm afraid is in that list which is why confusion still abounds. This conflicts with the more generally accepted definition of cycle time in manufacturing, which corresponds to the target rate of working expressed as Takt Time, and is the reciprocal of Delivery Rate. In other words this confusion of terminology is at least as old as the reference Little and Graves cite: Factory Physics by Hopp and Spearman (1st edition:1996).

Useful background, but the message to me is still: "Don't use Cycle Time in Kanban!".

References:

[1] Little, J. D. C and S. C. Graves (2008). Little's Law, pp 81-100, in D. Chhajed and TJ. Lowe (eds.) Building Intuition: Insights From Basic Operations Management Models and Principles. doi: 10.1007/978-0-387 -73699-0, (c) Springer Science + Business Media, LLC http://web.mit.edu/sgraves/www/papers/Little%27s%20Law-Published.pdf

[2] Hopp, W. J. and M. L. Spearman (2000). Factory Physics: Foundations of Manufacturing Management, 2nd (ed.), Irwin McGraw Hill, New York, NY.

### Does your Definition of Done allow known defects?

Is it just me or do you also find it odd that some teams have clauses like this in their definition of done (DoD)?
... the Story will contain defects of level 3 severity or less only ... Of course they don't mean you have to put minor bugs in your code - that really would be mad - but it does mean you can sign the Story off as "Done"if the bugs you discover in it are only minor (like spelling mistakes, graphical misalignment, faults with easy workarounds, etc.). I saw DoDs like this some time ago and was seriously puzzled by the madness of it. I was reminded of it again at a meet-up discussion recently - it's clearly a practice that's not uncommon.

Let's look at the consequences of this policy.

Potentially for every User Story that is signed off as "Done" there could be several additional Defect Stories (of low priority) that will be created. It's possible that finishing a Story (with no additional user requirements) will result in an increase in…

### "Plan of Intent" and "Plan of Record"

Ron Lichty is well known in the Software Engineering community on the West Coast as a practitioner, as a seasoned project manager of many successful ventures and in a number of SIGs and conferences in which he is active. In spite of knowing Ron by correspondence over a long period of time it was only at JavaOne this year that we finally got together and I'm very glad we did.

Ron wrote to me after our meeting:

I told a number of people later at JavaOne, and even later that evening at the Software Engineering Management SIG, about xProcess. It really looks good. A question came up: It's a common technique in large organizations to keep a "Plan of Intent" and a "Plan of Record" - to have two project plans, one for the business partners and boss, one you actually execute to. Any support for that in xProcess?