The Smart Forecaster

 Pursuing best practices in demand planning,

forecasting and inventory optimization

Quants and Financial Meltdowns

I spend much of my time developing new quantitative methods for statistical forecasting, demand forecasting and inventory optimization. For me, this is an engaging way to contribute to society. But I know that the most prudent way to do algorithm development is to stand a little to the side and cast a skeptical eye on my own work.

The need for this skepticism was highlighted for me recently as I read Scott Patterson’s book The Quants: How a New Breed of Math Whizzes Conquered Wall Street and Nearly Destroyed It (Crown Publishing, 2010). This book reviewed the “quants” whose complex financial models were largely responsible for the financial meltdown in 2007. As I read along and thought “What was wrong with these guys?” I began to wonder if we supply chain quants were guilty of some of the same sins.

Models versus Instincts

Generally, the supply chain field has lagged behind finance in terms of the use of statistical models. My university colleagues and I are chipping away at that, but we have a long way to go. Some supply chains are quite technically sophisticated, but many, perhaps more, are essentially managed as much by gut instinct as by the numbers. Is this avoidance of analytics safer than relying on models?

What makes gut instinct dangerous is that it is so amorphous. Everyone who works long in a job develops instincts, but longevity is not the same as wisdom. It is possible to learn all the wrong lessons over a long career. It is also possible to miss the chance to learn the right lessons because certain informative scenarios may never arise in one person’s career. It is also possible to have good days and bad days; even gurus can mess up. Gut instinct is also anti-productive, since all decisions have to pass through that one gut, which becomes an enterprise chokepoint. And Golden Guts eventually reach their Golden Years and take their Golden Watch and go off into a Golden Sunset; at that point, whatever expertise had been present has walked out the door.

In contrast, models have certain advantages. Relative to gut instinct, models are:

  • Explicit: The theory of the supply chain operation is exposed for all to see.
  • Adaptive: Because the theory is visible, it can be reviewed, critiqued, tested against data, and evolved.
  • Consistent: Models may be more or less true, but they are not subject to day-to-day variability.
  • Comprehensive: At least potentially, models can accumulate a wide range of empirical experience, including scenarios never encountered during any one person’s career.
  • Instructive: Models are collections of relationships among variables. If the model’s “guts” are made visible, users can learn about those relationships.

Model Error

Nevertheless, despite all their virtues, models can also be wrong. In fact, that is a given. A constructive way to live with this is encoded in the famous aphorism by Dr. George Box, one of the best modelers of the last half century: “All models are wrong. Some are useful.”

The finance quants’ models were wrong by being oversimplified. They started with a quasi-religious belief in the efficiency of markets and developed statistical models that made certain assumptions that were more likely to be true of the physical world than the financial world. Among these were Normal distributions of changes in asset prices and independence of events across various corners of the market. They also assumed human rationality.

It should be a bit alarming that the Normal distribution and independence assumptions also underlie many of the models in supply chain software. In fact, there are alternative models of supply chain dynamics that do not require these simplifying assumptions, so this is an unnecessary risk being run by many, perhaps most, of the users of supply chain software.

But even with more robust and realistic model assumptions, there is no denying that model error is a constant risk. So, can you be victimized by your models? Of course you can.

Self-Protection: Look at the Data

Every supply chain professional who uses models, then, is subject to model risk. But unlike with decisions based on gut feel, decisions based on model calculations can be exposed and compared to real-world outcomes. Repeated checking is the best way to protect against model error, because it not only tests whether the model is realistic but also signals when it is time to update the model.

As noted above, a model is a set of functional relationships between key variables. Those relationships have parameters that tune the model to the current operating context. For instance, supply chain models often rely, in part, on estimates of demand volatility. Historical demand data are used to calculate numerical values for these parameters. If demand volatility changes, the model becomes obsolete and likely to produce inapt recommendations. Therefore, good practice demands frequent updates to model parameters.

Even when parameter values are current, there may still be trouble due to incorrect functional relationships. For example, consider the relationship between the mean and standard deviation of demand for spare parts. Generally speaking, the greater the average demand, the greater the demand volatility as measured by the standard deviation.

Now consider simplified “old school” models that describe spare part demand as a Poisson process. The Poisson process is widely useful and relatively simple, so it often shows up in Statistics 101 classes. Because of their relative simplicity, Poisson models are the white rats of supply chain analytics for spare parts, i.e., people do computer experiments and theory development based on the behavior of Poisson models of demand. For Poisson models, the standard deviation of demand equals the square root of the mean. However, when we look at our customers’ actual demand data, we discover that the actual relationship between the mean and standard deviation of demand is better described by a more general power-law relationship. Thus, the simple model may use accurate estimates of mean and standard deviation but still not accurately reflect their relationship. This in turn leads to incorrect recommendations about reorder points for spare parts. Checking real data is the best antidote to cavalier assumption-making.

 

What to Do Next

I do not sense that today’s supply chain models are on the brink of creating the kind of meltdown we saw in the start of the Great Recession. But those of us who are supply chain quants need to show more professional maturity than our financial colleagues. We need to not fall in love with our models, and we need to alert our customers to correct model hygiene.

So, model users, wash your hands frequently as we begin flu season, and wash your models thoroughly through hard data to be sure that the models you rely on are both up-to-date and grounded in reality. Both those steps will protect you from being victimized by your models and let you exploit their advantages over management by gut feel.

Appendix: Technical Tips

Supply chain analytics provide various types of outputs. In the realm of forecasting and demand planning, the obvious empirical check is to compare forecasts against the actual demand values that eventually reveal themselves. This same “forecast then check” approach can also be used in the generation of forecasts.  In the realm of inventory management, the models can build on forecasts to recommend policy choices, such as reorder points and order quantities or Min and Max values. There is a smart way to confirm the accuracy of recommendations of reorder points and Min’s.  See our blog The Right Forecast Accuracy Metric for Inventory Planning

 

Leave a Comment

Related Posts

The Supply Chain Blame Game:  Top 3 Excuses for Inventory Shortage and Excess

The Supply Chain Blame Game: Top 3 Excuses for Inventory Shortage and Excess

The supply chain has become the blame game for almost any industrial or retail problem. Shortages on lead time variability, bad forecasts, and problems with bad data are facts of life, yet inventory-carrying organizations are often caught by surprise when any of these difficulties arise. So, again, who is to blame for the supply chain chaos? Keep reading this blog and we will try to show you how to prevent product shortages and overstocking.

Recent Posts

  • Mature bearded mechanic in uniform examining the machine and repairing it in factoryPlanning for Consumable vs. Repairable Parts
    When deciding on the right stocking parameters for spare and replacement parts, it is important to distinguish between consumable and repairable parts. These differences are often overlooked by inventory planning software and can result in incorrect estimates of what to stock. Different approaches are required when planning for consumables vs. repairables. […]
  • Four Common Mistakes when Planning Replenishment TargetsFour Common Mistakes when Planning Replenishment Targets
    How often do you recalibrate your stocking policies? Why? Learn how to avoid key mistakes when planning replenishment targets by automating the process, recalibrating parts, using targeting forecasting methods, and reviewing exceptions. […]
  • Smart Software is pleased to introduce our series of webinars, offered exclusively for Epicor Users.Extend Epicor Kinetic’s Forecasting & Min/Max Planning with Smart IP&O
    Epicor Kinetic can manage replenishment by suggesting what to order and when via reorder point-based inventory policies. The problem is that the ERP system requires that the user either manually specify these reorder points, or use a rudimentary “rule of thumb” approach based on daily averages. In this article, we will review the inventory ordering functionality in Epicor Kinetic, explain its limitations, and summarize how to reduce inventory, and minimize stockouts by providing the robust predictive functionality that is missing in Epicor. […]
  • Scenario based Forecasting vs EquationsScenario-based Forecasting vs. Equations
    Traditionally, software has served as a delivery vehicle for equations. This is fine, as far as it goes. But we at Smart Software think you would do better by trading in your equations for scenarios. Learn why Scenario-based planning helps planners better manage risk and create better outcomes. […]
  • Extend Microsoft 365 BC and NAV with Smart IP&OExtend Microsoft 365 BC and NAV with Smart IP&O
    Microsoft 365 BC and NAV can manage replenishment by suggesting what to order and when via reorder point-based inventory policies. The problem is that the ERP system requires that the user manually specify these reorder points and/or forecasts. In this article, we will review the inventory ordering functionality in Microsoft BC & NAV, explain its limitations, and summarize how to reduce inventory, and minimize stockouts by providing the robust predictive functionality that is missing in Dynamics 365. […]

    Inventory Optimization for Manufacturers, Distributors, and MRO

    • Blanket Orders Smart Software Demand and Inventory Planning HDBlanket Orders
      Our customers are great teachers who have always helped us bridge the gap between textbook theory and practical application. A prime example happened over twenty years ago, when we were introduced to the phenomenon of intermittent demand, which is common among spare parts but rare among the finished goods managed by our original customers working in sales and marketing. This revelation soon led to our preeminent position as vendors of software for managing inventories of spare parts. Our latest bit of schooling concerns “blanket orders.” […]
    • Hand placing pieces to build an arrowProbabilistic Forecasting for Intermittent Demand
      The New Forecasting Technology derives from Probabilistic Forecasting, a statistical method that accurately forecasts both average product demand per period and customer service level inventory requirements. […]
    • Engineering to Order at Kratos Space – Making Parts Availability a Strategic Advantage
      The Kratos Space group within National Security technology innovator Kratos Defense & Security Solutions, Inc., produces COTS s software and component products for space communications - Making Parts Availability a Strategic Advantage […]
    • wooden-figures-of-people-and-a-magnet-team-management-warehouse inventoryManaging the Inventory of Promoted Items
      In a previous post, I discussed one of the thornier problems demand planners sometimes face: working with product demand data characterized by what statisticians call skewness—a situation that can necessitate costly inventory investments. This sort of problematic data is found in several different scenarios. In at least one, the combination of intermittent demand and very effective sales promotions, the problem lends itself to an effective solution. […]