Do you have hidden conservatism built into your MRP?

One of the more common axioms in the manufacturing world is “Inventory is Evil”, because too much inventory can only lead to problems. The most common problems often sited are the cash consumed purchasing the inventory, increased warehouse costs to store excess inventory, higher obsolescence write-offs when a particular part is classified as obsolete or inactive, and extra manpower to count, review and manage the inventory.

So, how does a company, even a well-managed one, get into inventory trouble in the first place? Two obvious reasons include poor forecasting by sales and unexpected inventory obsolescence due to a technology change. However, to list all the potential reasons for high inventory levels is beyond the scope of this narrative as the intent is to help you to discover ways to reduce your inventory levels without making overhauling your current basic processes; in other words quick fixes that can have dramatic results.

One of operations’ key responsibilities is to ensure the appropriate level of inventory is on-hand to support sales. With this in mind there are four important points to consider:

  1. The main tool that operations use to recommend inventory purchases is Material Requirements Planning (MRP).
  2. The basic assumptions that drive purchase recommendations by the MRP are forecasted total sales, sales orders already in backlog, bill-of-materials (BOM), vendor and in-house lead times, and safety stock levels.
  3. The basic stereotypical trait of most manufacturing people is risk avoidance
  4. The general philosophy of most companies, particularly companies experiencing high growth (e.g. start-ups), is to not lose a sale due to lack of inventory. In addition, it is my experience from working for manufacturing companies in fast paced Silicon Valley, a manufacturing manager or purchasing employee is much more likely to get fired for missing a sale than ending up with excess inventory. Playing it safe thereby becomes a basic survival strategy.

Since the BOM, including yield percentages, always needs be accurate as possible and backlog is a known quantity, operations have only a few remaining ways to insert some conservatism (i.e., playing it safe) into the MRP output. They can increase the demand side of the calculation by nudging the Sales forecast up a bit or increase the supply requirements by ratcheting up either the lead times or safety stock levels of all purchased and manufactured inventory.

The analysis and impact of safety stock is pretty straightforward so I won’t spend much time here, but lead time is a bit more challenging and therefore probably warrants a more frequent review than safety stock levels. In fact, Supply Chain Digest published an article on May 4, 2006 titled ‘The Impact of Lead Time Variability’ highlighting “Preliminary Research out of Georgia Tech finds there’s a lot more variability on inbound deliveries than many companies may realize” and as Georgia Tech’s Dr. Donald Ratliff noted that “not only does lead time variability impact a variety of supply chain cost and performance metrics, the impact of variability is actually greater the more efficient a company’s supply chain is.”

That is a rather thought provoking mouthful. In essence, even though lead time variability impact is more prevalent than believed, the impact is greater on well-run supply chains.

Given that a very common way to be conservative in inventory planning is to increase lead times and the implications on lead time variability per the Georgia Tech study, it warrants a more frequent review than it probably is receiving today. To complicate matters just a bit, there are numerous places where lead times can be used or “hidden”. However, a great place to start is the lead time for the receiving department to receive inventory and then place the item into its appropriate stock location in the warehouse. Depending on your particular situation, this particular lead time might even be considered superfluous, because if there is an urgent need for specific material, manufacturing will not only be aware of its arrival at the dock, they will find a way to expedite the material out of receiving and onto the production floor.

Coming from my finance background, conservatism does have its place. However, the impact of being conservative should at the very least be understood by all so that any conservatism becomes a conscious decision by the executive team. For example, let’s assume that the annual COGS for a company is $10,000,000 and the inventory level is $2,500,000. This equates to an inventory turn of 4.0 or having 91.25 days of inventory on hand. If the company has a 1 day lead time for material receipts, then by eliminating this lead time, will by definition, reduce inventory to 90.25 days in inventory. This reduction will result in an inventory being reduced by 1.1% / $110,000 as the company’s MRP will now calculate the inventory purchase requirements based on a shorter lead time.

Bottom line: both lead times and safety stock are typically set and reviewed infrequently. However, as their impact can be a significant drain on cash and a cause of an increase in inventory obsolescence expenses when parts are classified as inactive, they both should be reviewed and analyzed for appropriateness and accuracy at least once a year to ensure that all inventory conservatism is known and understood by all so that appropriate action can be taken.

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Written by: Bob Swedroe, Former President and CEO at Expandable Software, Inc.

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  • Great content!

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