So, after huddling in a secret executive planning session, the decision is made to mandate an increase in safety stock quantities.  After all, ACME's sales are going up, therefore more inventory is needed to support the customers. Someone even has a computer printout telling them how much more safety stock is needed. But, increasing safety stock levels doesn't make the capacity or supplier problems go away. In fact, increasing safety stock now only adds even more pressure to these problems.

Safety Stock: What does it do?

The Master Schedule report in figure 2  shows what happens when the safety stock quantity is increased from 0 (as seen in figure 1) to 100 units (as seen in figure 2).

After the change to safety stock is made, expedite action messages are created recommending moving all re-supply orders (MPS) up three weeks. To satisfy the forecast and increase safety stock at the same time, the re-supply orders must be finished three weeks earlier than originally planned. This increases total demand (required capacity and material) on

manufacturing resources at the exact time they are suffering from a shortage of resources due to the increased sales rate. Lead times are being crunched, material shortages are increasing and manufacturing gets further behind. Because this increased demand exceeds demonstrated capacity and causes mass confusion on the production floor, every production job is late but some are now really HOT. Manufacturing has started a daily hot list meeting to find out what is really needed and when.

What went wrong with this approach? Let's look at the Master Schedule in figure 1 again. In periods 4,7, and 10 we have resupply orders of 120 units each scheduled to be finished and available for sale. The 120 units are set by the order quantity. The key is changing the safety stock quantity doesn't affect the quantity produced, it changes the timing of resupply. Increasing safety stock tells production they must finish work sooner and, admittedly it's uncommon, decreasing safety stock tells production to finish work later. The order quantity or lot size dictates how much you will get and safety stock affects when it should be finished.

Now  the manufacturing people that were struggling to produce enough for shipping the increased customers' orders must also cut the lead time in half for the next 3 or more MPS orders. Did things just get better? Of course not, and in most companies things just got a lot worse. No longer can manufacturing rely on the Master Schedule to establish priorities, due dates and quantities. This is where a lot of companies would throw in the towel and revert to the informal methods of hot lists, daily late order meeting, and material shortage meetings.

Paradigm Shift

The paradigm shift for changing safety stock quantities is that the most effective time to increase safety stock is when you can make more than you can sell, not the reverse - although in a lot of companies the exact opposite is the most common practice. The second paradigm shift is that safety stock can be a barrier or shield to root cause problems in the business. It is not uncommon to see companies spend hundreds of hours and thousands of dollars debating, calculating, updating and expediting production departments and suppliers to increase safety stock but put little or no effort in reducing forecast variability. Many companies have found that investing in improving forecasting and sales planning activities has a better and more permanent payback than playing the safety stock game.

Key Points

Some of the key points to keep in mind when changing safety stock levels are: