Keep the balance with 8 inventory management viewpoints
Many companies initiate projects on inventory optimisation, when inventories are too large and/or delivery performance is too low. All too often, companies face both challenges at the same time and, consequently, there is a need to find a balanced approach, where net working capital and delivery performance are equally in focus.
Based on experience from a wide range of projects carried out across companies, industries and nationalities, the following 8 inventory management viewpoints have been defined for the purpose of simplifying and improving the way in which we manage our inventories:
The 8 viewpoints can be used to set up a process at your company that will allow you to manage the inventory in a balanced and continuous manner. Do not be surprised if you achieve inventory reductions of 10 – 40 % – and at the same time even increase delivery performance.
Where P signifies the calculated inventory management parameters, C signifies the cycle stock (e.g. due to batch production), T signifies the planners’ trust in the inventory management viewpoints, and S signifies the strategic stock. Notice T 2, which signifies that a high degree of confidence in the inventory management viewpoints lowers the total inventories – squared! Thus, the planners’ trust in the established inventory management viewpoints and processes is a significant factor in the size of the inventory.
In the following, the individual inventory management viewpoints are presented.
It almost sounds trivial, but the first inventory management viewpoint is actually to understand your inventory. The inventory consists of a number of components and factors, all of which affect the inventory in one way or another. As not all factors affect the inventory in the same direction, there are, of course, a number of trade-offs to consider. It is therefore important to break down the inventories into different components; safety stock, cycle stock, stock in transit, strategic stock etc. and, subsequently, to list the factors or “levers” that affect the individual stock components. The breakdown and visibility of the net working capital in the individual stock components and the associated levers make it possible to launch dedicated efforts within selected and prioritised areas.
Your company’s inventory management should take place at different levels. At the operational level, the focus is to ensure that customer orders are met and inventories are increased or reduced in time. This is a process, which takes place daily or weekly.
At the tactical level, which takes place monthly or quarterly, the focus is to ensure that the product range is segmented in order to apply a differentiated approach (elaborated in viewpoint no. 5).
Finally, at the strategic level, the balance between delivery performance and net working capital is defined. At this level, it is not enough to ask “Sales”. Instead, the top management must make the decision based on a weighting between delivery performance and net working capital, which brings us to the third inventory management viewpoint.
In theory, the optimal delivery performance is calculated based on the costs of excess stock (storage costs) compared to the costs of not being able to deliver (lost sales and goodwill). In practice, however, this is rarely an option, because the total costs of lost sales are very difficult to estimate, and the trade-off against storage costs is an impossibility. Instead, you should determine the balance based on the following:
The trade-off between delivery performance and net working capital should always be balanced between Sales, Finance and Production/Procurement.
By making the relation between delivery performance and net working capital visible, it is possible to simulate the consequences of the costs of a specific delivery performance.
Both demand and supply for the individual products vary over the delivery time offered by suppliers and/or internal production. From the time an order (supply) is placed until it reaches the warehouse, the number of customer orders may change – a larger or smaller number of orders than expected may be placed.
Furthermore, the quantities that are ordered for supply may differ from what you actually receive and, finally, delivery time may vary as well. Demand variability increases over delivery time. The further you look into the future, the more uncertain is the demand. It is therefore important to understand the magnitude and consequences of the uncertainty over delivery time in order to balance the right amount of safety stock.
You can easily use your big inventory management hammer and treat everything relating to inventory management as nails, but, in many cases, the result is ugly. It is therefore important to segment your products in the inventory on the basis of the predictability/variance (forecast error, sales variance etc.) and order frequency.
The four segments are controlled based on different methods, which ensure that you can spend your time on products that require extra attention and free up time from products that do not require much attention (in your ERP or planning system).
It can be complicated to calculate safety stocks on the basis of uncertainty in the supply chain. As a consequence, many safety stocks are defined based simply on order sizes, inventory budgets or gut feelings. As the delivery performance is typically in focus, it may take some time for you to realise that the amount of net working capital is unnecessarily high (you will realise this when the CFO taps you on the shoulder!), and when that happens, an inventory optimisation project is initiated with the purpose of reducing the inventories. Subsequently, the inventories are gradually increased again as delivery performance fails for the individual products.
By using formulas (the fill rate method is recommended) for calculating the safety stocks for the stable products, the right/required balance is achieved between the net working capital and the delivery performance, cf. the management decision in inventory management viewpoint no. 2 – a balance that requires maintenance!
For unstable products, it is rarely possible to use a standard formula for calculating safety stocks. Spare parts are a typical example of products that are not stable. It can be very difficult to predict when a need arises. Consequently, it is difficult to safeguard against uncertainty.
For some of these products (valuable/important), there may be an agreement to have a strategic safety stock. The classic example is the ship, which is in port because of a lack of spare parts. In that case, the daily rate of the ship can often justify additional net working capital for the spare parts. For other less valuable products, simulations are a great tool for determining the safety stocks.
The eighth and final inventory management viewpoint is as simple as the first viewpoint, which is to use the 7 other viewpoints in a standardised process – preferably with system support. You should not settle for immediate benefits from a thorough inventory clean-up. After a clean-up, the inventory will increase once again, and the delivery performance will gradually decrease as time goes by. Instead, inventory management should be a well-defined process that is repeated week after week, month after month, year after year. The process can be defined based on the planning triangle described in inventory management viewpoint no. 2.
When you, in the near future, start to use the 8 inventory management viewpoints, you will discover that some products need higher inventories, while others need lower inventories compared with the current status. The majority of products will, in all probability, be in the latter category. With this in mind, you must remember that an increase in the inventory can take place immediately, while a decrease in the inventory will take place over a period of time (e.g. as demand gradually reduces the inventory). In any case, you will, over time, reap significant benefits by following the 8 inventory management viewpoints.
During the entire process, it is important to keep the inventory formula (T 2!) in mind. No inventory management viewpoints or processes are advanced enough to keep a planner from overwriting the result, if he/she does not have confidence in the result.
This article is originally authored by Christian Guldager.
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