Wednesday, August 6, 2008

Inventory costs

Inventory costs consist of:
• Cost of money - The cost of capital to the company or, in some cases the "opportunity cost" or return that might be earned on the money by applying it productively elsewhere.
• Obsolescence - The risk of inventory never being used, or needing rework to make it usable, needs to be factored into the cost of owning INVENTORY. In theory (and practice), the larger the inventory is, and the longer it is held, the more likely engineering changes, customer preferences and technological changes will render that inventory unusable. In the clothing industry, it is not uncommon to see inventories depreciate as much as 90% when styles change. Certain portions of the electronics industry have problems with inventory becoming obsolete very quickly, due to technological changes.
• Shrinkage - A portion of inventory becomes unavailable to the owner due to loss, damage, theft or spoilage. The longer inventory is there and the more there is, the more likely this is to happen. Steps to prevent it only raise carrying costs in other areas, such as security, climate control, better control systems, recruiting policies, etc.
• Quality Factors - Allowances for yield, attrition, scrap and rework. This is really more of a function of the process than the amount of inventory invested and is more related to throughput, but is sometimes included as part of the aggregate inventory carrying cost.
• Technological or Price Obsolescence - Prices don't always go up. In fact, in industries such as electronics, prices often plummet due to constantly improving designs, product and process technology improvements. Therefore, it is desirable to minimize inventories in high-risk areas.
• Taxes - There are two dimensions to this: 1) in some areas, a tax is levied on inventories, so the more inventory, the more tax is paid. 2) inventory is regarded as an asset by most accounting and tax rules. Therefore, increasing inventories shows "profits" and profits are usually taxed, usually by multiple government entities.
• Insurance - The cost of carrying insurance on inventory needs to be considered, as well as insuring the space, equipment, people and other resources needed to control it.
• Space - Costly storage space sometimes occupies 25-30% of the total facility, when one considers raw material warehouses, stockrooms, work-in-process storage, receiving, shipping, outside warehouses, MRB and residual storage areas. Inventory reduction campaigns can help companies avoid the need to move to large facilities, or permit them to shut down or cut back existing facilities.
• Manpower - All of this inventory needs people to order, receive inspect, record, move, count, store, retrieve, post it to the ledger, etc. People are the largest or second largest expense (behind material) for most manufacturers.
• Record Keeping Systems - Software, procedures, equipment and paper must be used to track and control inventory.
• Material Handling/Storage Equipment - Conveyors, fork lifts, bar code readers, scales, automated storage and retrieval systems, trucks, carts, bins, racks, shelves must all be purchased, leased, maintained and cared for.
• Physical Inventories, Reconciliations - Must be conducted to ensure that inventories are properly accounted for and maintained.
• Transportation - Must be provided to move inventory in and out of the facility, to vendors, within the facility, to different workstations and storage areas.
• Energy - Heat, light, humidity control, air conditioning, refrigeration and fuel must be consumed to make all this happen.
• Inappropriate Lot Sizing - In inventory formulae, the carrying cost of inventory is often expressed as a flat percentage of the inventory value, for convenience of computations, but that is an oversimplification of reality. For instance, consider material handling/storage costs. Just because a dollar of inventory is added, doesn't mean that carrying costs go up, say, $.02. In reality the costs would not usually go up in a direct proportion at all, but only when we had to pay for an additional expense, or make the next capital investment in equipment or space to accommodate the inventory. So actually, most of these costs are step functions, rather than continuous curves.
We urge caution in the use of so-called EOQ (Economic Order Quantity) formulae in planning. While these can be useful guidelines in some cases, they can easily go awry and are hypersensitive to changes in carrying costs and order costs, which are usually no more than guesstimates, at best. We smile in amusement at PhD's made or lost on the study of such arcane calculations, often failing to consider basic realities such as; how much space and money do we have, anyway? You can refer to Paul's book, Production & Inventory Management in the Technological Age, pages 137 to 139 for a detailed explanation of why this lot sizing method is weak and should be used with caution.

• Supply variation—refers to the reliability of the supplier to deliver the desired units in the needed quantity, at the right time, at an acceptable quality level. If this can't be done reliably, then companies tend to carry a buffer (safety) stock to make up for the deficiencies in the supply system.

• Demand variation - refers to the ability to reliably forecast what the customer will require (whether that is an internal or an external customer). Lower reliability tends to encourage buffer (safety) stocks.

• Defects —Extra inventory is often carried to allow for probable rejections. This is just a specialized form of safety stock for supply and demand buffering.

• Logistics constraints/transportation costs - This also sometimes falls under the heading of supply and demand variation and it certainly can affect it. For example, one of our clients transports parts by ocean freight to a plant in Portugal, or at least they do that if they don't have to ship by air to get them there faster. Because ships traveling between economical ports only leave every few weeks, a 20 or 40 foot long container is the most practical shipping size. A certain amount of time is required for packing, transportation to the terminal, Loading, transport, unloading, customs and transport to the consignee. These are very real logistics constraints that must be built into the "pipeline" portion of the inventory model.

No comments: