Inventory overstocking can harm both financial stability and operational efficiency. When an organization is overstocked, it ties up capital in excess inventory that might not sell, increasing storage costs and the risk of inventory obsolescence. Additionally, the funds used to purchase the excess inventory could have been better invested in other areas of the business, such as marketing or research and development. Overstocking also hampers cash flow, as money is locked in stock rather than available for immediate operational needs. Managing inventory effectively is critical for maintaining a healthy balance sheet and ensuring that resources are optimally allocated. Here is an in-depth exploration of the main causes of overstocking, their implications, and possible solutions.

1 Inaccurate Demand Forecasting

One of the primary causes of overstocking is inaccurate demand forecasting. When businesses rely on outdated forecasting methods or insufficient data, they can easily overestimate demand, leading to overstocking. A prime example is the clothing industry, where fashion trends can change rapidly. A well-known fashion brand recently faced challenges after overestimating demand for a new clothing line based on flawed data analysis, leading to unsold inventory.

To address this issue, companies can implement new technologies that automatically select the best forecasting methods for the data, incorporating trends and seasonal patterns to ensure accuracy. By improving forecasting accuracy, businesses can better align their inventory with actual demand, leading to more precise inventory management and fewer overstock scenarios. For instance, a Hardware retailer using Smart Demand Planner reduced forecasting errors by 15%, demonstrating the potential for significant improvement in inventory management​​​​.

2 Improper Inventory Management

Effective inventory management is fundamental to prevent overstocking. Without accurate systems to track inventory levels, businesses might order excess stock and incur higher expenses. This issue often stems from reliance on spreadsheets or inefficient ERP systems that lack real-time data integration.

State-of-the-art technologies provide real-time visibility into inventory levels, allowing businesses to automate and optimize reordering processes.  A large electric utility company faced challenges in maintaining service parts availability without overstocking, managing over 250,000 part numbers across a diverse network of power generation and distribution facilities. The company replaced its outdated system with Smart IP&O and integrated it in real-time with their Enterprise Asset Management (EAM) system. Smart IP&O enabled the utility to use “what-if” scenarios, creating digital twins of alternate stocking policies and simulating performance across key performance indicators, such as inventory value, service levels, fill rates, and shortage costs. This allowed the utility to make targeted adjustments to their stocking parameters, which were then deployed to their EAM system, driving optimal replenishments of spare parts.

The outcome was significant: a $9 million reduction in inventory, freeing up cash and valuable warehouse space while sustaining target service levels of over 99%​

3 Overly Optimistic Sales Projections

Businesses, especially those in growth phases, may predict higher sales than they achieve, leading to excess inventory intended to meet anticipated demand that never materializes. An example of this is the recent case with an electric vehicle manufacturer that projected high sales for its truck but faced production delays and lower-than-expected demand, resulting in an overstock of components and parts. This miscalculation led to increased storage costs and strained financial resources.

Another automotive aftermarket company struggled to forecast intermittently demanded parts accurately, frequently resulting in overstocking and stockouts.  Using AI-driven technology enabled the company to significantly reduce backorders and lost sales, with fill rates improving from 93% to 96% within just three months. By leveraging Smart IP&O forecasting technologies, the company could generate accurate estimates of cumulative demand over lead times, providing better visibility of potential demand scenarios. This allowed for optimized inventory levels, reducing storage costs and improving financial efficiency by aligning inventory with actual demand​.

4 Bulk Purchasing Discounts

The appeal of cost savings from bulk purchases can prompt businesses to buy more than needed, tying up capital and storage space. This often leads to storage challenges when excess stock is ordered to secure a discount.

To address this challenge, businesses should weigh the benefits of bulk discounts against the costs of holding excess inventory. Next-generation technology can help identify the most cost-effective purchasing strategy by balancing immediate savings with long-term storage costs. By implementing Smart IP&O, MNR could accurately forecast inventory requirements and optimize its inventory management processes. This led to an 8% reduction in parts inventory, reaching a high customer service level of 98.7% and reducing inventory growth for new equipment from a projected 10% to only 6%.

5 Seasonal Demand Fluctuations

Difficulty in aligning inventory with seasonal demand can result in surplus stock once the peak sales period ends. Toy manufacturers, for example, might produce too many holiday-themed toys only to face low demand after the holidays. The fashion industry frequently experiences similar challenges, with certain styles becoming obsolete as seasons change. The latest technologies can help businesses anticipate seasonal demand shifts and adjust inventory levels accordingly. By analyzing past sales data and predicting future trends, businesses can better prepare for seasonal fluctuations, minimize overstocking risk, and improve inventory turnover.

6 Supplier Lead Time Variability

Unreliable supplier lead times can lead to overstocking as a buffer against delays. If lead times improve or demand decreases unexpectedly, businesses may have excess inventory. For example, an auto parts distributor might stockpile components to mitigate supplier delays, only to find lead times improving suddenly.

Advanced technology can help by providing real-time data and predictive analytics to manage lead time variability better. These tools allow companies to dynamically adjust their orders, reducing the need for excessive safety stock.

7 Inadequate Inventory Policies

Outdated or incorrect inventory policies, such as faulty Min/Max settings, can lead to over-ordering.  However, using Modern technology to regularly review and update inventory policies ensures they align with current business needs and market conditions. By keeping policies up-to-date, businesses can reduce the risk of overstocking due to procedural errors. A recent case study demonstrated how a major retailer used Smart IP&O to revise inventory policies, resulting in a 15% reduction in overstock​​.

8 Promotions and Marketing Campaigns

Misalignment between marketing efforts and actual customer demand can cause businesses to overestimate the impact of promotions, resulting in unsold inventory. For example, a cosmetics company might overproduce a limited edition product, expecting high demand that doesn’t materialize. Leveraging Smart IP&O can help align marketing initiatives with realistic demand expectations, avoiding excess stock. By integrating marketing plans with demand forecasts, businesses can optimize their promotional strategies to better match actual customer interest.

9 Fear of Stockouts

Companies often maintain higher inventory levels to avoid stockouts, which can lead to lost sales and unhappy customers. This fear can drive businesses to overstock as a safety net, especially in industries where customer satisfaction and retention are crucial. A notable example comes from a large retail chain that significantly increased its inventory of household goods to avoid stockouts. While this strategy initially helped meet customer demand, it later resulted in excess inventory as consumer purchasing patterns stabilized. This overstocking contributed to a profit drop of nearly 90% in the second quarter, largely due to markdowns and the clearing of excess stock.

To mitigate such situations, businesses can utilize advanced inventory planning and optimization tools to provide accurate demand forecasts. For instance, a leading electronics manufacturer used Smart IP&O solution to reduce inventory levels by 20% without impacting service levels, effectively reducing costs while maintaining customer satisfaction by ensuring they had the right amount of stock on hand​​​​.

10 Overcompensation for Supply Chain Issues

Businesses may overstock to safeguard against ongoing supply chain disruptions, but this can lead to storage issues. For instance, a tech company might stockpile components to avoid potential supply chain hiccups, resulting in surplus inventory and increased costs. Advanced systems can help businesses better anticipate and respond to supply chain challenges, balancing the need for safety stock with the risk of overstocking. A technology firm used Smart IP&O to streamline its inventory strategy, reducing excess stock by 20% while maintaining supply chain resilience​​.

11 Long Lead Times and Unreliable Suppliers

Prolonged lead times and unreliable suppliers can lead businesses to order more stock than needed to cover potential supply gaps. However, less critical Items that are forecasted to achieve very high service levels represent opportunities to reduce inventory.  By targeting lower service levels on less critical items, inventory will be “right size” over time to the new equilibrium, decreasing holding costs and the value of inventory on hand. A major public transit system reduced inventory by more than $4,000,000 while improving service levels using our cutting-edge technology.

12 Lack of Real-Time Inventory Visibility

Without real-time insights into inventory, businesses often order more stock than necessary, leading to inefficiencies and increased costs. Smart IP&O enabled Seneca companies to model demand at each stocking location and, using service level-driven planning, determine how much to stock to achieve the service level we require.  By running and comparing different scenarios, they can easily define and update optimal stocking policies for each tech support rep and stockrooms.

The software has provided field technicians with evidence they did not have before, showing them their actual consumption, frequency of part use, and rationale for stocking policies, using 90% as the targeted service level norm.  Field technicians have embraced its use, with significant results:  “Zero Turns” inventory has dropped from $400K to under $100K, “First Fix Rate” exceeds 90%, and total inventory investment has decreased by more than 25%, from $11 million to $ 8 million.

In conclusion, overstocking seriously threatens business profitability and efficiency, leading to increased storage costs, tied-up capital, and potential obsolescence of goods. These issues can strain resources and limit a company’s ability to respond to market changes. However, overstocking can be effectively managed by understanding its causes, such as inaccurate demand forecasting, prolonged lead times, and unreliable suppliers. Implementing robust AI-driven solutions like Smart IP&O can help businesses optimize inventory levels, reduce excess stock, and enhance operational efficiency. By leveraging advanced forecasting and inventory optimization tools, companies can find the right balance in meeting customer demand and minimizing inventory-related costs.

Forecast-based inventory management, or MRP (Material Requirements Planning) logic, is a forward-planning methodology for managing inventory. This method ensures that businesses can meet demand without overstocking, which ties up capital, or understocking, which can lead to lost sales and dissatisfied customers.

By anticipating demand and adjusting inventory levels accordingly, this approach helps maintain the right balance between having enough stock to meet customer needs and minimizing excess inventory costs. Businesses can optimize operations, reduce waste, and improve customer satisfaction by predicting future needs. Let’s break down how this works.

Core Concepts of Forecast-Based Inventory Management

Inventory Dynamics Models: Inventory dynamics models are fundamental to understanding and managing inventory levels. The simplest model, known as the “sawtooth” model, demonstrates inventory levels decreasing with demand and replenishing just in time. However, real-world scenarios often require more sophisticated models. By incorporating stochastic elements and variability, such as Monte Carlo simulations, businesses can account for random fluctuations in demand and lead time, providing a more realistic forecast of inventory levels.

IP&O platform enhances inventory dynamics modeling through advanced data analytics and simulation capabilities. By leveraging AI and machine learning algorithms, our IP&O platform can predict demand patterns more accurately, adjusting models in real time based on the latest data. This leads to more precise inventory levels, reducing the risk of stockouts and overstocking.

Determining Order Quantity and Timing: Effective inventory management requires knowing when and how much to order. This involves forecasting future demand and calculating the lead time for replenishing stock. By predicting when inventory will hit safety stock levels, businesses can plan their orders to ensure continuous supply.

Our latest tools excel at optimizing order quantities and timing by utilizing predictive analytics and AI. These systems can analyze vast amounts of data, including historical sales and market trends. By doing so, they provide more accurate demand forecasts and optimize reorder points, ensuring inventory is replenished just in time without excess.

Calculating Lead Time: Lead time is the period from placing an order to receiving the stock. It varies based on the availability of components. For example, if a product is assembled from multiple components, the lead time will be determined by the component with the longest lead time.

Smart AI-driven solutions enhance lead time calculation by integrating with supply chain management systems. These systems track supplier performance, and historical lead times, to provide more accurate lead time estimates. Additionally, smart technologies can alert businesses to potential delays, allowing for proactive adjustments to inventory plans.

Safety Stock Calculation: Safety stock acts as a buffer to protect against variability in demand and supply. Calculating safety stock involves analyzing demand variability and setting a stock level that covers most potential scenarios, thus minimizing the risk of stockouts.

IP&O technology significantly improves safety stock calculation through advanced analytics. By continuously monitoring demand patterns and supply chain variables, smart systems can dynamically adjust safety stock levels. Machine learning algorithms can predict demand spikes or drops and adjust safety stock accordingly, ensuring optimal inventory levels while minimizing holding costs.

The Importance of Accurate Forecasting in Inventory Management

Accurate forecasting is key for minimizing forecast errors, which can lead to excess inventory or stockouts. Techniques such as utilizing historical data, enhancing data inputs, and applying advanced forecasting methods help achieve better accuracy. Forecast errors can have significant financial implications: over-forecasting results in excess inventory while under-forecasting leads to missed sales opportunities. Managing these errors through systematic tracking and adjusting forecasting methods is crucial for maintaining optimal inventory levels.

Safety stock ensures that businesses meet customer needs even if actual demand deviates from the forecast. This cushion protects against unforeseen demand spikes or delays in replenishment. Accurate forecasting, effective error management, and strategic use of safety stock enhance forecast-based inventory management. Companies can understand inventory dynamics, determine the right order quantities and timing, calculate accurate lead times, and set appropriate safety stock levels.

Using state-of-the-art technology like IP&O provides significant advantages by offering real-time data insights, predictive analytics, and adaptive models. This leads to more efficient inventory management, reduced costs, and improved customer satisfaction. Overall, IP&O empowers businesses to plan better and respond swiftly to market changes, ensuring they maintain the right inventory balance to meet customer needs without incurring unnecessary costs.

​Demand planning and statistical forecasting software play a pivotal role in effective business management by incorporating features that significantly enhance forecasting accuracy. One key aspect involves the utilization of smoothing-based or extrapolative models, enabling businesses to quickly make predictions based solely on historical data. This foundation rooted in past performance is crucial for understanding trends and patterns, especially in variables like sales or product demand. Forecasting software goes beyond mere data analysis by allowing the blending of professional judgment with statistical forecasts, recognizing that forecasting is not a one-size-fits-all process. This flexibility enables businesses to incorporate human insights and industry knowledge into the forecasting model, ensuring a more nuanced and accurate prediction.

Features such as forecasting multiple items as a group, considering promotion-driven demand, and handling intermittent demand patterns are essential capabilities for businesses dealing with diverse product portfolios and dynamic market conditions.  Proper implementation of these applications empowers businesses with versatile forecasting tools, contributing significantly to informed decision-making and operational efficiency.

Extrapolative models

Our demand forecasting solutions support a variety of forecasting approaches including extrapolative or smoothing-based forecasting models, such as exponential smoothing and moving averages.  The philosophy behind these models is simple: they try to detect, quantify, and project into the future any repeating patterns in the historical data.

  There are two types of patterns that might be found in the historical data:

• Trend
• Seasonality

These patterns are illustrated in the following figure along with random data.

Illustrating trending, seasonal, and random time series data

If the pattern is a trend, then extrapolative models such as double exponential smoothing and linear moving average estimates the rate of increase or decrease in the level of the variable and project that rate into the future.

If the pattern is seasonality, then models such as Winters and triple exponential smoothing estimate either seasonal multipliers or seasonal add factors and then apply these to projections of the nonseasonal portion of the data.

Very often, especially with retail sales data, both trend and seasonal patterns are involved. If these patterns are stable, they can be exploited to give very accurate forecasts.

Sometimes, however, there are no obvious patterns, so that plots of the data look like random noise. Sometimes patterns are clearly visible, but they change over time and cannot be relied upon to repeat. In these cases, the extrapolative models don’t try to quantify and project patterns. Instead, they try to average through the noise and make good estimates of the middle of the distribution of data values. These typical values then become the forecasts.  Sometimes, when users see a historical plot with lots of ups and downs they are concerned when the forecast doesn’t replicate those ups and downs. Normally, this should not be a reason for concern.  This occurs when the historical patterns aren’t strong enough to warrant using a forecasting method that would replicate the pattern.  You want to make sure your forecasts don’t suffer from the “wiggle effect” that is described in this blog post.

Past as a predictor of the future

The key assumption implicit in extrapolative models is that the past is a good guide to the future. This assumption, however, can break down. Some of the historical data may be obsolete. For example, the data might describe a business environment that no longer exists. Or, the world that the model represents may be ready to change soon, rendering all the data obsolete. Because of such complicating factors, the risks of extrapolative forecasting are lower when forecasting only a short time into the future.

Extrapolative models have the practical advantage of being cheap and easy to build, maintain and use. They require only accurate records of past values of the variables you need to forecast. As time goes by, you simply add the latest data points to the time series and reforecast. In contrast, the causal models described below require more thinking and more data. The simplicity of extrapolative models is most appreciated when you have a massive forecasting problem, such as making overnight forecasts of demand for all 30,000 items in inventory in a warehouse.

Judgmental adjustments

Extrapolative models can be run in a fully automatic mode with Demand Planner with no intervention required. Causal models require substantive judgment for wise selection of independent variables. However, both types of statistical models can be enhanced by judgmental adjustments. Both can profit from your insights.

Both causal and extrapolative models are built on historical data. However, you may have additional information that is not reflected in the numbers found in the historical record. For instance, you may know that competitive conditions will soon change, perhaps due to price discounts, or industry trends, or the emergence of new competitors, or the announcement of a new generation of your own products. If these events occur during the period for which you are forecasting, they may well spoil the accuracy of purely statistical forecasts. Smart Demand Planner’ graphical adjustment feature lets you include these additional factors in your forecasts through the process of on- screen graphical adjustment.

Be aware that applying user adjustments to the forecast is a two-edged sword. Used appropriately, it can enhance forecast accuracy by exploiting a richer set of information. Used promiscuously, it can add additional noise to the process and reduce accuracy. We advise that you use judgmental adjustments sparingly, but that you never blindly accept the predictions of a purely statistical forecasting method.  It is also very important to measure forecast value add.  That is, the value added to the forecast process by each incremental step.  For example, if you are applying overrides based on business knowledge, it is important to measure whether those adjustments are adding value by improving forecast accuracy.  Smart Demand Planner supports measurement of forecast value add by tracking every forecast considered and automating the forecast accuracy reports. You can select statistical forecasts, measure their errors, and compare them to the overridden ones.  By doing so, you inform the forecasting process so that better decisions can be made in the future. 

Multiple-level forecasts

Another common situation involves multiple-level forecasting, where there are multiple items being forecast as a group or there may even be multiple groups, with each group containing multiple items. We will generally call this type of forecasting Multilevel Forecasting. The prime example is product line forecasting, where each item is a member of a family of items, and the total of all the items in the family is a meaningful quantity.

For example, as in the following figure, you might have a line of tractors and want forecasts of sales for each type of tractor and for the entire tractor line.

Illustrating multiple-level product forecasts

 Smart Demand Planner provides Roll Up/Roll Down Forecasting. This function is crucial for obtaining comprehensive forecasts of all product items and their group total. The Roll Down/Roll Up method within this feature offers two options for obtaining these forecasts:

Roll Up (Bottom-Up): This option initially forecasts each item individually and then aggregates the item-level forecasts to generate a family-level forecast.

Roll Down (Top-Down): Alternatively, the roll-down option starts by forming the historical total at the family level, forecasts it, and then proportionally allocates the total down to the item level.

When utilizing Roll Down/Roll Up, you have access to the full array of forecast methods provided by Smart Demand Planner at both the item and family levels. This ensures flexibility and accuracy in forecasting, catering to the specific needs of your business across different hierarchical levels.

Forecasting research has not established clear conditions favoring either the top-down or bottom-up approach to forecasting. However, the bottom-up approach seems preferable when item histories are stable, and the emphasis is on the trends and seasonal patterns of the individual items. Top-down is normally a better choice if some items have very noisy history or the emphasis is on forecasting at the group level. Since Smart Demand Planner makes it fast and easy to try both a bottom-up and a top- down approach, you should try both methods and compare the results.  You can use Smart Demand Planner’s “Hold back on Current”  feature in the “Forecast vs. Actual” to test both approaches on your own data and see which one yields a more accurate forecast for your business. 

Measuring the accuracy of forecasts is an undeniably important part of the demand planning process. This forecasting scorecard could be built based on one of two contrasting viewpoints for computing metrics. The error viewpoint asks, “how far was the forecast from the actual?” The accuracy viewpoint asks, “how close was the forecast to the actual?” Both are valid, but error metrics provide more information.

Accuracy is represented as a percentage between zero and 100, while error percentages start at zero but have no upper limit. Reports of MAPE (mean absolute percent error) or other error metrics can be titled “forecast accuracy” reports, which blurs the distinction.  So, you may want to know how to convert from the error viewpoint to the accuracy viewpoint that your company espouses.  This blog describes how with some examples.

Accuracy metrics are computed such that when the actual equals the forecast then the accuracy is 100% and when the forecast is either double or half of the actual, then accuracy is 0%. Reports that compare the forecast to the actual often include the following:

• The Actual
• The Forecast
• Unit Error = Forecast – Actual
• Absolute Error = Absolute Value of Unit Error
• Absolute % Error = Abs Error / Actual, as a %
• Accuracy % = 100% – Absolute % Error

Look at a couple examples that illustrate the difference in the approaches. Say the Actual = 8 and the forecast is 10.

Unit Error is 10 – 8 = 2

Absolute % Error = 2 / 8, as a % = 0.25 * 100 = 25%

Accuracy = 100% – 25% = 75%.

Now let’s say the actual is 8 and the forecast is 24.

Unit Error is 24– 8 = 16

Absolute % Error = 16 / 8 as a % = 2 * 100 = 200%

Accuracy = 100% – 200% = negative is set to 0%.

In the first example, accuracy measurements provide the same information as error measurements since the forecast and actual are already relatively close. But when the error is more than double the actual, accuracy measurements bottom out at zero. It does correctly indicate the forecast was not at all accurate. But the second example is more accurate than a third, where the actual is 8 and the forecast is 200. That’s a distinction a 0 to 100% range of accuracy doesn’t register. In this final example:

Unit Error is 200 – 8 = 192

Absolute % Error = 192 / 8, as a % = 24 * 100 = 2,400%

Accuracy = 100% – 2,400% = negative is set to 0%.

Error metrics continue to provide information on how far the forecast is from the actual and arguably better represent forecast accuracy.

We encourage adopting the error viewpoint. You simply hope for a small error percentage to indicate the forecast was not far from the actual, instead of hoping for a large accuracy percentage to indicate the forecast was close to the actual.  This shift in mindset offers the same insights while eliminating distortions.

Summary

Smart Software’s advanced supply chain analytics exploits multiple advanced methods. Two of the most important are “statistical bootstrapping” and “Monte Carlo simulation”. Since both involve lots of random numbers flying around, folks sometimes get confused about which is which and what they are good for. Hence, this note. Bottom line up front: Statistical bootstrapping generates demand scenarios for forecasting. Monte Carlo simulation uses the scenarios for inventory optimization.

Bootstrapping

Bootstrapping, also called “resampling” is a method of computational statistics that we use to create demand scenarios for forecasting. The essence of the forecasting problem is to expose possible futures that your company might confront so you can work out how to manage business risks. Traditional forecasting methods focus on computing “most likely” futures, but they fall short of presenting the full risk picture. Bootstrapping provides an unlimited number of realistic what-if scenarios.

Bootstrapping does this without making unrealistic assumptions about the demand, i.e., that it is not intermittent, or that it has a bell-shaped distribution of sizes. Those assumptions are crutches to make the math simpler, but the bootstrap is a procedure,  not an equation, so it doesn’t need such simplifications.

For the simplest demand type, which is a stable randomness with no seasonality or trend, bootstrapping is dead easy. To get a reasonable idea of what a single future demand value might be, pick one of the historical demands at random. To create a demand scenario, make multiple random selections from the past and string them together. Done. It is possible to add a little more realism by “jittering” the demand values, i.e., adding or subtracting a bit of additional randomness to each one, but even that is simple.

Figure 1 shows a simple bootstrap. The first line is a short sequence of historical demand for an SKU. The following lines show scenarios of future demand created by randomly selecting values from the demand history. For instance, the next three demand might be (0, 14, 6), or (2, 3, 5), etc.

Figure 1: Example of demand scenarios generated by a simple bootstrap

Higher frequency operations such as daily forecasting bring with them more complex demand patterns, such as double seasonality (e.g., day-of-week and month-of-year) and/or trend. This challenged us to invent a new generation of bootstrapping algorithms. We recently won a US Patent for this breakthrough, but the essence is as described above.

Monte Carlo Simulation

Monte Carlo is famous for its casinos, which, like bootstrapping, invoke the idea of randomness. Monte Carlo methods go back a long way, but the modern impetus came with the need to do some hairy calculations about where neutrons would fly when an A-bomb explodes.

The essence of Monte Carlo analysis is this: “Our problem is too complicated to analyze with paper-and-pencil equations. So, let’s write a computer program that codes the individual steps of the process, put in the random elements (e.g., which way a neutron shoots away), wind it up and watch it go. Since there’s a lot of randomness, let’s run the program a zillion times and average the results.”

Applying this approach to inventory management, we have a different set of randomly occurring events: e.g., a demand of a given size arrives on a random day, a replenishment of a given size arrives after a random lead time, we cut a replenishment PO of a given size when stock drops to or below a given reorder point. We code the logic relating these events into a program. We feed it with a random demand sequence (see bootstrapping above), run the program for a while, say one year of daily operations, compute performance metrics like Fill Rate and Average On Hand inventory, and “toss the dice” by re-running the program many times and averaging the results of many simulated years. The result is a good estimate of what happens when we make key management decisions: “If we set the reorder point at 10 units and the order quantity at 15 units, we can expect to get a service level of 89% and an average on hand of 21 units.” What the simulation is doing for us is exposing the consequences of management decisions based on realistic demand scenarios and solid math. The guesswork is gone.

Figure 2 shows some of the inner workings of a Monte Carlo simulation of an inventory system in four panels. The system uses a Min/Max inventory control policy with Min=10 and Max=25. No backorders are allowed: you have the good or you lose the business. Replenishment lead times are usually 7 days but sometimes 14. This simulation ran for one year.

The first panel shows a complex random demand scenario in which there is no demand on weekends, but demand generally increases each day from Monday to Friday. The second panel shows the random number of units on hand, which ebbs and flows with each replenishment cycle. The third panel shows the random sizes and timings of replenishment orders coming in from the supplier. The final panel shows the unsatisfied demand that jeopardizes customer relationships. This kind of detail can be very useful for building insight into the dynamics of an inventory system.

Figure 2: Details of a Monte Carlo simulation

Figure 2 shows only one of the countless ways that the year could play out. Generally, we want to average the results of many simulated years. After all, nobody would flip a coin once to decide if it were a fair coin. Figure 3 shows how four key performance metrics (KPI’s) vary from year to year for this system. Some metrics are relatively stable across simulations (Fill Rate), but others show more relative variability (Operating Cost= Holding Cost + Ordering Cost + Shortage Cost). Eyeballing the plots, we can estimate that the choices of Min=10, Max=25 leads to an average Operating cost of around $3,000 per year, a Fill Rate of around 90%, a Service Level of around 75%, and an Average On Hand of about 10

Figure 3: Variation in KPI’s computed over 1,000 simulated years

In fact, it is now possible to answer a higher level of management question. We can go beyond “What will happen if I do such-and-such?” to “What is the best thing I can do to achieve a fill rate of at least 90% for this item at the lowest possible cost?” The mathemagic  behind this leap is yet another key technology called “stochastic optimization”, but we’ll stop here for now. Suffice it to say that Smart’s SIO&P software can search the “design space” of Min and Max values to automatically find the best choice.