Negative inventory

Negative inventory effects on planning systems.

In addition to understanding the sources of negative inventory balances, it's also very important to understand their effects on planning and execution systems. In most planning systems, a negative item-level balance is treated the same as positive demand. Basically your system will tell you to make or buy more to offset the negative balance. Obviously this is a problem when a large negative balance occurs but can also create serious problems with small negative balances under certain conditions. For example, if you have an item that is set up as an "order as needed" item, meaning that you do not want to order or stock any unless you have actual demand (orders), an errant adjustment that drives the balance to   -5 will result in a recommended buy of 5 pieces. Since "order as needed" is often associated with very slow moving or obsolete items, you may have just added to an obsolescence problem. In a manufacturing environment using MRP, a negative balance of a single end-item, will result in demand cascading throughout the bill of material structure, potentially resulting in unnecessary orders for hundreds of lower-level items. This is what occurs if your system handles negative balances as would normally be expected. Some programs, either purposely or due to poor programming practices, may not execute properly if they encounter a negative balance. They may ignore the records with negative balances or simply "blow up" because they weren't designed to incorporate negative balances into their calculations. Though there are valid reasons for not wanting a program to execute if it encounters a negative balance, there are also potential problems with this logic. You may actually need to take action, but because the calculations were suspended due to a negative balance, you did not get the information needed to initiate an action. Due to the complexities of demand-planning systems, especially MRP/DRP systems, there is no "best" way to handle negative balances within the programming. Execution systems such as warehouse management systems and manufacturing execution systems can also have problems with negative balances. While you may not be willing to modify your systems to handle negative balances in a specific way, you should at least understand what your systems are doing when they encounter negative balances. Ultimately, avoidance of negative balances in the first place is the best solution. However, since perfection is pretty tough to achieve, you should have a backup plan. Since most planning systems still operate in batch mode (run nightly or on weekends) you can eliminate conflicts by resolving all negative balances prior to running these programs. With execution systems that are more likely to run real-time, you don't have this same luxury. Fortunately, the impact on execution systems is generally less dramatic than on planning systems. I'd once more like to emphasize the importance of thinking through your actions when trying to "fix" negative balances. A "run the report and adjust them all up" mentality will most certainly cause problems. Remember, you should not make adjustments to timing-related negative balances, you should only correct location-level problems with a location transfer program, and you should try to correct other negative balances by entering an offsetting transaction in the same program that created the problem.   Item-level negative balances Item-level negative balances, on the other hand, may be affecting planning system. These can occur from a variety of transactional mistakes. Over-reporting scrap quantities, cycle count adjustment errors, over-reporting production when using backflushing, overissuing materials to production, duplicate transactions, and overissuing inventory to shipping orders are just some examples of errors that can create item-level negative balances.   Correcting negative balances The reason it's so important to make the distinction as to the type of negative balance (timing, location-level, or item-level) is to ensure that your subsequent actions to "correct" the negative balance don't result in more serious inventory problems. If you were to adjust up a negative balance caused by a timing issue, you would create an inventory problem since, once the other transaction goes through, you will now be overstating your inventory by the amount of the adjustment. The same is true if you were to adjust up a negative location-level balance. Where your item-level balance was previously accurate, your adjustment would now result in overstatement of your item-level inventory. Item-level negative balances not related to timing issues should also be carefully investigated before taking any actions. Usually you will want to identify the transaction that caused the negative balance and use the same transaction program to enter an offsetting transaction. This is especially true in manufacturing environments since the incorrectly executed transaction may have also created errors with other items (such as with backflushing transactions). Despite the seemingly complex aspects of negative inventory balances, they are actually very easy to manage. They are easy to identify since all you need is a report that shows any items with a quantity on hand less than zero. In addition, it is usually very easy to track down the source of the error since the errant transaction is usually the transaction that brought the inventory balance below zero (if not, it very likely occurred within a very short timeframe prior to the negative balance being created).

Optimizing Safety Stock

Optimizing Safety Stock
Optimizing Safety Stock levels by calculating the magical balance of minimal inventory while meeting variable customer demand is sometimes described as the Holy Grail of inventory management (ok, forecasting is probably the true holy grail but I thought this sounded good). Many companies look at their own demand fluctuations and assume that there is not enough consistency to predict future variability. They then fall back on the trial and error best guess weeks supply method or the over simplified 1/2 lead time usage method to manage their safety stock. Unfortunately, these methods prove to be less than effective in determining optimal inventory levels for many operations. If your goal is to reduce inventory levels while maintaining or increasing service levels you will need to investigate more complex calculations.
One of the most widely accepted methods of calculating safety stock uses the statistical model of Standard Deviations of a Normal Distribution of numbers to determine probability. This statistical tool has proven to be very effective in determining optimal safety stock levels in a variety of environments. The basis for this calculation is standardized, however, its successful implementation generally requires customization of the formula and inputs to meet the specific characteristics of your operation. Understanding the statistical theory behind the formula is necessary in correctly adapting it to meet your needs. Errors in implementation are usually the result of not factoring in variables which are not part of original statistical model

Terminology and calculations

The following is a list of the variables and the terminology used in this safety stock model:
Normal distribution. Term used in statistical analysis to describe a distribution of numbers in which the probability of an occurrence, if graphed, would follow the form of a bell shaped curve. This is the most popular distribution model for determining probability and has been found to work well in predicting demand variability based upon historical data.
Standard deviation. Used to describe the spread of the distribution of numbers. Standard deviation is calculated by the following steps:
determine the mean (average) of a set of numbers.
determine the difference of each number and the mean
square each difference
calculate the average of the squares
calculate the square root of the average.
You can also use Excel function STDEVPA to calculate standard deviation. In safety stock calculations, the forecast quantity is often used instead of the mean in determining standard deviation.
Lead time. Highly accurate lead times are essential in the safety stock/reorder point calculation. Lead time is the amount of time from the point at which you determine the need to order to the point at which the inventory is on hand and available for use. It should include supplier or manufacturing lead time, time to initiate the purchase order or work order including approval steps, time to notify the supplier, and the time to process through receiving and any inspection operations.
Lead-time demand. Forecasted demand during the lead-time period. For example, if your forecasted demand is 3 units per day and your lead time is 12 days your lead time demand would be 36 units.
Forecast. Consistent forecasts are also an essential part of the safety stock calculation. If you don't use a formal forecast, you can use average demand instead.
Forecast period. The period of time over which a forecast is based. The forecast period used in the safety stock calculation may differ from your formal forecast periods. For example, you may have a formal forecast period of four weeks while the forecast period you use for the safety stock calculation may be one week.
Demand history. A history of demand broken down into forecast periods. The amount of history needed depends on the nature of your business. Businesses with a lot of slower moving items will need to use more demand history to get an accurate model of the demand. Generally, the more history the better, as long as sales pattern remains the same.
Order cycle. Also called replenishment cycle, order cycle refers to the time between orders of a specific item. Most easily calculated by dividing the order quantity by the annual demand and multiplying by the number of days in the year.
Reorder point. Inventory level which initiates an order. Reorder Point = Lead Time Demand + Safety Stock.
Service level. Desired service level expressed as a percentage.
Service factor. Factor used as a multiplier with the Standard Deviation to calculate a specific quantity to meet the specified service level. I have included a service factor table below or you can use Excel function NORMSINV to convert service level percentage to service factor.
By Dave Piasecki

Optimizing Economic Order Quantity (EOQ)

Optimizing Economic Order Quantity (EOQ)
Inventory models for calculating optimal order quantities and reorder points have been in existence long before the arrival of the computer. When the first Model T Fords were rolling off the assembly line, manufacturers were already reaping the financial benefits of inventory management by determining the most cost effective answers to the questions of When? and How much?. Yes long before JIT, TQM, TOC, and MRP, companies were using these same (then unnamed) concepts in managing their production and inventory. I recently read Purchasing and Storing, a textbook that was part of a “Modern Business Course” at the Alexander Hamilton Institute in New York. The textbook published in 1931 (that’s right 1931) was essentially a how to book on inventory management in a manufacturing environment. If you’re wondering why I would want to read a 70-year-old business text, my answer would be that the fundamental concepts of managing a business change very little with time, and reading about these concepts in a vintage text is a great way to reinforce the value of the fundamentals. The occasional reference to “The War” (referring to WWI) also keeps it interesting and the complete absence of acronyms is refreshing. As you may have guessed, this 70-year-old book contained a section on Minimum Cost Quantity, which is what we now refer to as Economic Order Quantity (EOQ). I can imagine that in the 1930’s an accountant (or more likely a room full of accountants) would have calculated EOQ or other inventory related formulas one item at a time in a dimly lit office using the inventory books, a mechanical adding machine and a slide rule. Time consuming as this was, some manufacturers of the time recognized the financial benefits of taking a scientific approach to making these inventory decisions. So why is it that, in these days of advanced information technology, many companies are still not taking advantage of these fundamental inventory models? Part of the answer lies in poor results received due to inaccurate data inputs. Accurate product costs, activity costs, forecasts, history, and lead times are crucial in making inventory models work. Ironically, software advancements may also in part to blame. Many ERP packages come with built in calculations for EOQ which calculate automatically. Often the users do not understand how it is calculated and therefore do not understand the data inputs and system setup which controls the output. When the output appears to be "out of whack" it is simply ignored. This sometimes creates a situation in which the executives who had purchased the software incorrectly assume the material planners and purchasing clerks are ordering based upon the systems recommendations. I should also note that many operations will find these built-in EOQ calculations inadequate and in need of modifications to deal with the diversity of their product groups and processes. Corporate goals and strategies may sometimes conflict with EOQ. Measuring performance solely by inventory turns is one of the most prolific mistakes made in the name of inventory management. Many companies have achieved aggressive goals in increasing inventory turns only to find their bottom line has shrunk due to increased operational costs. EOQ is essentially an accounting formula that determines the point at which the combination of order costs and inventory carrying costs are the least. The result is the most cost effective quantity to order. In purchasing this is known as the order quantity, in manufacturing it is known as the production lot size. While EOQ may not apply to every inventory situation, most organizations will find it beneficial in at least some aspect of their operation. Anytime you have repetitive purchasing or planning of an item, EOQ should be considered. Obvious applications for EOQ are purchase-to-stock distributors and make-to-stock manufacturers, however, make-to-order manufacturers should also consider EOQ when they have multiple orders or release dates for the same items and when planning components and sub-assemblies. Repetitive buy maintenance, repair, and operating (MRO) inventory is also a good application for EOQ. Though EOQ is generally recommended in operations where demand is relatively steady, items with demand variability such as seasonality can still use the model by going to shorter time periods for the EOQ calculation. Just make sure your usage and carrying costs are based on the same time period. Doesn’t EOQ conflict with Just-In-Time? While I don’t want to get into a long discussion on the misconceptions of what Just-In-Time (JIT) is, I will address the most common misunderstanding in which JIT is assumed to mean all components should arrive in the exact run quantities “just in time” for the production run. JIT is actually a quality initiative with the goal of eliminating wasted steps, wasted labor, and wasted cost. EOQ should be one of the tools used to achieve this. EOQ is used to determine which components fit into this JIT model and what level of JIT is economically advantageous for your operation. As an example, let us assume you are a lawn equipment manufacturer and you produce 100 units per day of a specific model of lawn mower. While it may be cost effective to have 100 engines arrive on your dock each day, it would certainly not be cost effective to have 500 screws (1 days supply) used to mount a plastic housing on the lawn mower shipped to you daily. To determine the most cost effective quantities of screws or other components you will need to use the EOQ formula. The basic Economic Order Quantity (EOQ) formula is as follows:

By Dave Piasecki

Cycle Counting and Physical Inventories

Cycle Counting and Physical Inventories

So it's the end of the year and the warehouse workers and all the salaried employees are gathered together on a Saturday morning to perform the annual physical inventory. The coffee and donuts help to put color into the faces and cover up the odors enveloping those who had overindulged themselves the night before. People are wandering around not sure what they should be doing, when the boss walks in with stacks of reports, cards, and colored stickers and says "OK here's how this is going to work." By noon it's obvious that less than half the warehouse has been counted and the pizza lunch has left everyone with an enthusiasm deficit. At two o'clock, one by one, people start approaching the boss with the reasons as to why they have to leave. Suddenly the pressure increases on those remaining to get finished. Five o'clock and the last of the counters are abandoning ship, there's an enormous pile of paperwork marked "discrepancies" and several piles of product marked "unknown," "what's this?" and "needs to be identified." The boss surveys the scene and instructs the people in charge of investigating the overwhelming pile of discrepancies to "just make the adjustments, we need to get out of here." With some variations, this is how annual physical inventories are performed year after year. So what’s wrong with this process? Everything!!! You’ve just had a group of people with inadequate training and experience — most of them forced into being there on their day off — count your inventory, and have then made adjustments to your on-hand balances based on those counts without having the time to adequately investigate the variances. The final result likely being that half of the adjustments corrected previous inventory problems while the other half created new inventory problems on items that were correct prior to the inventory. In case it’s not obvious to you, I don’t like annual physical inventories. Counting inventories on a regular basis throughout the year (cycle counting) combined with a process for continuous improvement in inventory accuracy will prove a far better method for achieving accurate inventories. My definition of cycle counting tends to differ slightly from the generally accepted one. Most people think of cycle counting as regularly scheduled (usually daily) counting of product where you randomly count items based upon some type of predefined parameters. For example, inventory is broken down by ABC classifications and frequencies assigned such as A items counted 10 times/year, B items 5 times/year, and son on. I prefer to define cycle counting as any count program using regularly scheduled counts where you count less than the entire facility's inventory during each count. This includes a system that I’ve found to be highly effective, that is a hybrid of a physical inventory and a cycle count, where you’re counting all inventory within a physical area like a physical inventory, however, you are not counting the entire facility at one time. The next day you simply start where you left off the day before. Regularly scheduled physical inventories can be an effective way of counting inventory in smaller operations provided you are using trained counters and have adequate time to investigate the discrepancies prior to making adjustments. If your inventory is so extensive that you cannot adequately investigate the count discrepancies, you must break it down into some sort of a cycle count program. If you are running a successful and comprehensive cycle counting program, there is little benefit to performing an annual physical inventory. Unfortunately, many in the financial establishment still live in the Dark Ages when it comes to inventory counting and will try to tell you that “you must perform an annual physical”. Once again I’ll state that if you can count and adequately investigate count discrepancies in a single day, then go ahead and perform the physical. However, if your inventory is too extensive or if you are in a 24/7 operation, do not want to shut down operations, and feel confident in the accuracy of your cycle count program, you can pressure them into accepting some type of on-site audit instead. They generally don’t like it but they will do it.

By Dave Piasecki

Material Requirements PlanningInvatol

Material Requirements PlanningInvatol

Material Requirements Planning MRP for small to mid size companies. Material Requirements Planning MRP covers a wide range of activities which include some of the following:
- Time Phased Orders -- Requirements sorted by Vendor -- Incorporate In House and Vendor Lead Times -- Focus on On Time Delivery -- Reduce Inventory Levels -
Material requirements planning or MRP involves getting material on hand when needed for production or sales. The Material requirements planning document should provide four basic items of information, when to place order, how much to order, who to order from and when the items need to be on hand. While some companies don't use MRP, but rely on expediting to accomplish this, other companies use the min max stock level. Both these methods are costly and often fail to meet production or sales needs that a good Material Requirements Planning process can provide. The only practical way to provide Material Requirements Planning is by using some type of computerized MRP program. Material Requirements Planning can plan, schedule and reschedule materials as far into the future as required.
Each company has an overall goal and a strategy for achieving that goal, but within the company there are groups whose focus may seem to be in contradiction to the Materials Requirements Planning process. The marketing group wants to make sure there is enough inventory on hand to supply all customer request, the accounting group is charged with keeping cost down, which includes keeping inventory levels low. In the middle is the inventory control group. If the accounting group is quiet, the marketing group is demanding you increase inventory, if the marketing group is quiet the accounting group wants to know why you have so much inventory. A good Material Requirements Planning system will help the inventory group balance both requirements, provide product for customer requirements and keep inventory levels low.
The goal of the MRP or Material Requirements Planning document is to supply information that will enable the company to have enough inventory on hand to fulfill demand, (and no more) available only when needed, (and no sooner) at a quality level that meets specification, (but does not have to exceed it) and at the lowest price. A good MRP or Material Requirements Planning program can provide the basic needs of keeping inventory levels low and fulfilling customer expectatons for on time delivery.