Shaunak, a reader of LSS Academy, recently sent me the following question via email.
By the way, if you have ever have questions related to continuous improvement (lean and/or six sigma) feel free to email me. I will do my very best to answer.
I have very quick question for you.
The level of inventory keeps changing all day, so when we draw the current state map, how to count the inventory?
I mean to ask: suppose SWIP decided is 20 pieces. At a point in time, inventory can be anything between 0-20.
Should we consider inventory level at the time when we are drawing the map or should we take the average inventory for the shift?
This is one of the most common questions I hear when teaching value stream mapping.
Specifically, if we draw the current state value stream map on Monday, and there are 42 pieces of inventory between stations A and B… but then I come back Wednesday and there are now 68 pieces… which do I document on my value stream map – 42 or 68?
Two Approaches to Handling This Problem
As with anything, there are several ways to approach this problem. Allow me to share two approaches I have personally used.
- Document what you see on Monday (42) and aggressively move on towards your improved future state.
- Study the situation over the span of a few days or weeks and note the inventory levels as both a measure of central tendency (mean or median) and measure of dispersion (standard deviation or range).
Now, something else I’d like to mention is if you find yourself dealing with highly erratic inventory levels day in and day out you likely have other, more serious, problems besides not knowing what to draw on your value stream map.
Don’t Agonize Over Details
So, instead of spending hours and hours, even days, worrying about what inventory levels to write on the VSM just document something and get focused on improving flow, preferably one piece at a time, while also working on finding a way to smooth and/or level production via heijunka.
You see, in the end, moving towards the ideal state is what lean is all about and this cannot happen if we never take that first, and often scary, step towards the improved future state.
How Would You Handle This Situation?
With this all said, I am curious on how you handle this situation.
How do you react to varying inventory levels on your value stream maps? And for those in transactional worlds, even health care worlds, how do you deal with varying queue times?