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SCDigest Expert Insight: Supply Chain by Design

About the Author

Dr. Michael Watson, one of the industry’s foremost experts on supply chain network design and advanced analytics, is a columnist and subject matter expert (SME) for Supply Chain Digest.

Dr. Watson, of Northwestern University, was the lead author of the just released book Supply Chain Network Design, co-authored with Sara Lewis, Peter Cacioppi, and Jay Jayaraman, all of IBM. (See Supply Chain Network Design – the Book.)

Prior to his current role at Northwestern, Watson was a key manager in IBM's network optimization group. In addition to his roles at IBM and now at Northwestern, Watson is director of The Optimization and Analytics Group.

By Dr. Michael Watson

November 20 , 2013

What Makes a Good Inventory Buffer

If You Don’t Set your Inventory Buffers Correctly, it Will Increase Other Costs

Dr. Watson Says:

...The problem is that you have too much variability in the supply chain.
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Several years ago, one of my students who worked for a Fortune 100 company told the following story:

"The CEO mandated that the company reduce inventory by 75%.  This was easy, the company reduced inventory by 75%.  Problem solved, right?  Well, production the next month slipped by 50%.  Then, because they couldn’t ship orders, sales slipped by 75%.  They raised the inventory levels back up and everyone pretended the program never existed."

What went wrong?  Inventory was playing a key role in buffering variability and the CEO didn’t realize it.  When something went wrong in the supply chain, customers had to wait.

If the CEO had realized that there are three ways to buffer variability in your supply chain, he might not have tried to just reduce inventory.   As a reminder, you can buffer variability as follows: 


With inventory—when something goes wrong, you have a pile of extra inventory cover your needs


With capacity—when something goes wrong, you run overtime or pay for expediting to cover your needs


With time—when something goes wrong, you simply make your customer wait.


Previous Columns by Dr. Watson

The Three Use Cases for Data Scientists

Learn Python, PuLP, Jupyter Notebooks, and Network Design

EOQ Model and the Hidden Costs of Fixed Costs

CSCMP Edge - Nike Quote: "It is All an Art Project Until you Get it on Someone's Feet"

Supply Chain by Design: Why Business Leaders should think of AI as an Umbrella Term


A common supply chain buffer is inventory.  This is one of the main reasons you have a distribution center that stocks inventory.

One problem that companies run into is that their inventory buffer is too small—like the mandated 50% smaller pile the CEO above recommended.  When this happens, you will unintentionally revert to the other two buffers—you will expedite shipments or make your customers wait longer (and not book the sales).

One way to think about a good buffer is that it should be large enough so that customers who are served by this buffer don’t see any problems.  You may have terrible suppliers, unreliable plants, and unpredictable customers.   But, if you easily satisfy demand from inventory, your customers are none the wiser and think you are running a good supply chain.

You also don’t want to have too much inventory.  If you do, you are tying up too much cash—cash you could use for new investments—and have too much in extra carrying costs. 

To properly set your inventory levels, you should account for the major components that drive inventory levels such as expected demand and variability; supplier lead time and lead time variability, supplier service levels, your desired service level, and your batch sizes.  With these inputs, there is commercial software that will calculate the proper inventory levels for you—that is the easy part. 

One hard part is maintaining a good inventory buffer with the right level of inventory.  For this, I’ve seen that is important for customers to put a strong process in place to reset the inventory levels every month or quarter. 

The other hard part is that at some point, someone will complain that you have too much inventory.  If you are setting your inventory levels correctly, the inventory isn’t the problem.  The problem is that you have to too much variability in the supply chain.  To reduce the inventory, go after the variability first and then reduce inventory later.

Final Thoughts

For more information, Supply Chain Digest recently ran a VideoCast on inventory that does a nice job of discussing how lead time variability drives safety stock.



Recent Feedback

Inventory is cash and it eats liquidity. It adds to the cost of controllable efforts to handle.  The priority is space cost and infrastructure cost, along with less inventory and more control of liquidity and other obsolence and costs. Maybe hire one more resource for better planning when purchasing inventory.

Jan, 24 2014