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Little’s Law: Isn’t It a Linear Relationship?

“Here’s the bottom line: the number one driver for shipping products quicker is by focusing on the important ones and killing the unimportant ones.”

“You might be thinking: ‘True, but couldn’t we also increase the average completion rate’? You’re right, but the impact of doing that is much lower than reducing the TIP (things-in-process) — that is, influencing the average completion rate is rather difficult and is often a function of available resources, scope creep, market demands, and changes, etc.”

– Pete Abilla, Nov 2006, Little’s Law for Product Development

A few weeks back Arne Roock (see his posts here), a fellow kanban/lean-agile practitioner, pinged me with a question related to Little’s Law and utilization. Paraphrasing, essentially it was “Queuing theory states that the speed of networks decreases dramatically (non-linearly) as utilization increases more than 80%. But according to Little‘s Law (given a stable system), Lead Time increases linearly if we increase WIP (which increases utilization). Why doesn’t Little’s Law show lead time going up exponentially from a certain point on (ex. past 80% utilization)?” This resulted in exchanging a couple of emails discussing the use of Little’s Law, and why and how in the software development context an increase in work-in-progress could result in a non-linear increase in lead time. This post captures and reflects some of the thoughts we shared. My assumption is you’ve wondered too about similar questions. If so, I hope you’ll find this post interesting and helpful.