Probabilistic vs. Deterministic Order Planning

The Smart Forecaster

 Pursuing best practices in demand planning,

forecasting and inventory optimization

Consider the problem of replenishing inventory. To be specific, suppose the inventory item in question is a spare part. Both you and your supplier will want some sense of how much you will be ordering and when. And your ERP system may be insisting that you let it in on the secret too.

Deterministic Model of Replenishment

The simplest way to get a decent answer to this question is to assume the world is, well, simple. In this case, simple means “not random” or, in geek speak, “deterministic.” In particular, you pretend that the random size and timing of demand is really a continuous drip-drip-drip of a fixed size coming at a fixed interval, e.g., 2, 2, 2, 2, 2, 2… If this seems unrealistic, it is. Real demand might look more like this: 0, 1, 10, 0, 1, 0, 0, 0 with lots of zeros, occasional but random spikes.

But simplicity has its virtues. If you pretend that the average demand occurs every day like clockwork, it is easy to work out when you will need to place your next order, and how many units you will need.  For instance, suppose your inventory policy is of the (Q,R) type, where Q is a fixed order quantity and R is a fixed reorder point. When stock drops to or below the reorder point R, you order Q units more. To round out the fantasy, assume that the replenishment lead time is also fixed: after L days, those Q new units will be on the shelf ready to satisfy demand.

All you need now to answer your questions is the average demand per day D for the item. The logic goes like this:

  1. You start each replenishment cycle with Q units on hand.
  2. You deplete that stock by D units per day.
  3. So, you hit the reorder point R after (Q-R)/D days.
  4. So, you order every (Q-R)/D days.
  5. Each replenishment cycle lasts (Q-R)/D + L days, so you make a total of 365D/(Q-R+LD) orders per year.
  6. As long as lead time L < R/D, you will never stock out and your inventory will be as small as possible.

Figure 1 shows the plot of on-hand inventory vs time for the deterministic model. Around Smart Software, we refer to this plot as the “Deterministic Sawtooth.” The stock starts at the level of the last order quantity Q. After steadily decreasing over the drop time (Q-R)/D, the level hits the reorder point R and triggers an order for another Q units. Over the lead time L, the stock drops to exactly zero, then the reorder magically arrives and the next cycle begins.

Figure 1 Deterministic model of on-hand inventory

Figure 1: Deterministic model of on-hand inventory

 

This model has two things going for it. It requires no more than high school algebra, and it combines (almost) all the relevant factors to answer the two related questions: When will we have to place the next order? How many orders will we place in a year?

Probabilistic Model of Replenishment

Not surprisingly, if we strip away some of the fantasy from the deterministic model, we get more useful information. The probabilistic model incorporates all the messy randomness in the real-world problem: the uncertainty in both the timing and size of demand, the variation in replenishment lead time, and the consequences of those two factors: the chance of stock on hand undershooting the reorder point, the chance that there will be a stockout, the variability in the time until the next order, and the variable number of orders executed in a year.

The probabilistic model works by simulating the consequences of uncertain demand and variable lead time. By analyzing the item’s historical demand patterns (and excluding any observations that were recorded during a time when demand may have been fundamentally different), advanced statistical methods create an unlimited number of realistic demand scenarios. Similar analysis is applied to records of supplier lead times. Combining these supply and demand scenarios with the operational rules of any given inventory control policy produces scenarios of the number of parts on hand. From these scenarios, we can extract summaries of the varying intervals between orders.

Figure 2 shows an example of a probabilistic scenario; demand is random, and the item is managed using reorder point R = 10 and order quantity Q=20. Gone is the Deterministic Sawtooth; in its place is something more complex and realistic (the Probabilistic Staircase). During the 90 simulated days of operation, there were 9 orders placed, and the time between orders clearly varied.

Using the probabilistic model, the answers to the two questions (how long between orders and how many in a year) get expressed as probability distributions reflecting the relative likelihoods of various scenarios. Figure 3 shows the distribution of the number of days between orders after ten years of simulated operation. While the average is about 8 days, the actual number varies widely, from 2 to 17.

Instead of telling your supplier that you will place X orders next year, you can now project X ± Y orders, and your supplier knows better their upside and downside risks. Better yet, you could provide the entire distribution as the richest possible answer.

Figure 2 A probabilistic scenario of on-hand inventory

Figure 2 A probabilistic scenario of on-hand inventory

 

Figure 3 Distribution of days between orders

Figure 3: Distribution of days between orders

 

Climbing the Random Staircase to Greater Efficiency

Moving beyond the deterministic model of  inventory opens up new possibilities for optimizing operations. First, the probabilistic model allows realistic assessment of stockout risk. The simple model in Figure 1 implies there is never a stockout, whereas probabilistic scenarios allow for the possibility (though in Figure 2 there was only one close call around day 70). Once the risk is known, software can optimize by searching  the “design space” (i.e., all possible values of R and Q) to find a design that meets a target level of stockout risk at minimal cost. The value of the deterministic model in this more realistic analysis is that it provides a good starting point for the search through design space.

Summary

Modern software provides answers to operational questions with various degrees of detail. Using the example of the time between replenishment orders, we’ve shown that the answer can be calculated approximately but quickly by a simple deterministic model. But it can also be provided in much richer detail with all the variability exposed by a probabilistic model. We think of these alternatives as complementary. The deterministic model bundles all the key variables into an easy-to-understand form. The probabilistic model provides additional realism that professionals expect and supports effective search for optimal choices of reorder point and order quantity.

 

 

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    Increasing Revenue by Increasing Spare Part Availability

    The Smart Forecaster

     Pursuing best practices in demand planning,

    forecasting and inventory optimization

    Let’s start by recognizing that increased revenue is a good thing for you, and that increasing the availability of the spare parts you provide is a good thing for your customers.

    But let’s also recognize that increasing item availability will not necessarily lead to increased revenue. If you plan incorrectly and end up carrying excess inventory, the net effect may be good for your customers but will definitely be bad for you. There must be some right way to make this a win-win, if only it can be recognized.

    To make the right decision here, you have to think systematically about the problem. That requires that you use probabilistic models of the inventory control process.

     

    A Scenario

    Let’s consider a specific, realistic scenario. Quite a number of factors have an influence on the results:

    • The item: A specific low-volume spare part.
    • Demand mean: Averaging 0.1 units per day (so, highly “intermittent”)
    • Demand standard deviation: 0.35 units per day (so, highly variable or “overdispersed”).
    • Supplier average lead time: 5 days.
    • Unit cost: $100.
    • Holding cost per year as % of unit cost: 10%.
    • Ordering cost per PO cut: $25.
    • Stockout consequences: Lost sales (so, a competitive market, no backorders).
    • Shortage cost per lost sale: $100.
    • Service level target: 85% (so, 15% chance of a stockout in any replenishment cycle).
    • Inventory control policy: Periodic-review/Order-up-to (also called at (T,S) policy)

     

    Inventory Control Policy

    A word about the inventory control policy. The (T,S) policy is one of several that are common in practice. Though there are other more efficient policies (e.g., they don’t wait for T days to go by before making adjustment to stock), (T,S) is one of the simplest and so it is quite popular. It works this way: Every T days, you check how many units you have in stock, say X units. Then you order S-X units, which appear after the supplier lead time (in this case, 5 days). The T in (T,S) is the “order interval”, the number of days between orders; the S is the “order-up-to level”, the number of units you want to have on hand at the start of each replenishment cycle.

    To get the most out of this policy, you must wisely pick values of T and S. Picking wisely means you cannot win by guessing or using simple rule-of-thumb guides like “Keep an average of 3 x average demand on hand.”  Poor choices of T and S hurt both your customers and your bottom line. And sticking too long with choices that were once good can result in poor performance should any of the factors above change significantly, so the values of T and S should be recalculated now and then.

    The smart way to pick the right values of T and S is to use probabilistic models encoded in advanced software. Using software is essential when you have to scale up and pick values of T and S that are right for not one item but hundreds or thousands.

     

    Analysis of Scenario

    Let’s think about how to make money in this scenario. What’s the upside? If there were no expenses, this item could generate an average of $3,650 per year: 0.1 units/day x 365 days x $100/unit. Subtracted from that will be operating costs, comprised of holding, ordering and shortage costs. Each of those will depend on your choices of T and S.

    The software provides specific numbers: Setting T = 321 days and S = 40 units will result in average annual operating costs of $604, giving an expected margin of $3,650 – $604 = $3,046. See Table 1, left column. This use of software is called “predictive analytics” because it translates system design inputs into estimates of a key performance indicator, margin.

    Now think about whether you can do better. The service level target in this scenario is 85%, which is a somewhat relaxed standard that is not going to turn any heads. What if you could offer your customers a 99% service level? That sounds like a distinct competitive advantage, but would it reduce your margin? Not if you properly adjust the values of T and S.

    Setting T = 216 days and S = 35 units will reduce average annual operating costs to $551 and increase expected margin to $3,650 – $551 = $3,099. See Table 1, right column. Here is the win-win we wanted: higher customer satisfaction and roughly 2% more revenue. This use of the software is called “sensitivity analysis” because it shows how sensitive the margin is to the choice of service level target.

    Software can also help you visualize the complex, random dynamics of inventory movements. A by-product of the analysis that populated Table 1 are graphs showing the random paths taken by stock as it decreases over a replenishment cycle. Figure 1 shows a selection of 100 random scenarios for the scenario in which the service level target is 99%. In the figure, only 1 of the 100 scenarios resulted in a stockout, confirming the accuracy of the choice of order-up-to-level.

     

    Summary

    Management of spare parts inventories is often done haphazardly using gut instinct, habit, or obsolete rule-of-thumb. Winging it this way is not a reliable and reproducible path to higher margin or higher customer satisfaction. Probability theory, distilled into probability models then encoded in advanced software, is the basis for coherent, efficient guidance about how to manage spare parts based on facts: demand characteristics, lead times, service level targets, costs and the other factors. The scenarios analyzed here illustrate that it is possible to achieve both higher service levels and higher margin. A multitude of scenarios not shown here offer ways to achieve higher service levels but lose margin. Use the software.

    Scenarios with different service level targets

    Stock on hand during one replenishment cycle

     

     

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      About Smart Software, Inc.

      Founded in 1981, Smart Software, Inc. is a leader in providing businesses with enterprise-wide demand forecasting, planning and inventory optimization solutions.  Smart Software’s demand forecasting and inventory optimization solutions have helped thousands of users worldwide, including customers at mid-market enterprises and Fortune 500 companies, such as  Disneyland Resorts, Hitachi, Otis Elevator, Metro-North Railroad, and American Red Cross.  Smart Inventory Planning & Optimization gives demand planners the tools to handle sales seasonality, promotions, new and aging products, multi-dimensional hierarchies, and intermittently demanded service parts and capital goods items.  It also provides inventory managers with accurate estimates of the optimal inventory and safety stock required to meet future orders and achieve desired service levels.  Smart Software is headquartered in Belmont, Massachusetts and can be found on the World Wide Web at www.smartcorp.com.

       

      SmartForecasts and Smart IP&O are registered trademarks of Smart Software, Inc.  All other trademarks are the property of their respective owners.


      For more information, please contact Smart Software, Inc., Four Hill Road, Belmont, MA 02478.
      Phone: 1-800-SMART-99 (800-762-7899); FAX: 1-617-489-2748; E-mail: info@smartcorp.com

       

       

      Four Useful Ways to Measure Forecast Error

      The Smart Forecaster

       Pursuing best practices in demand planning,

      forecasting and inventory optimization

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      In this video, Dr. Thomas Willemain, co-Founder and SVP Research, talks about improving forecast accuracy by measuring forecast error. We begin by overviewing the various types of error metrics: scale-dependent error, percentage error, relative error, and scale-free error metrics. While some error is inevitable, there are ways to reduce it, and forecast metrics are necessary aids for monitoring and improving forecast accuracy. Then we will explain the special problem of intermittent demand and divide-by-zero problems. Tom concludes by explaining how to assess forecasts of multiple items and how it often makes sense to use weighted averages, weighting items differently by volume or revenue.

       

      Four general types of error metrics 

      1. Scale-dependent error
      2. Percentage error
      3. Relative error
      4 .Scale-free error

      Remark: Scale-dependent metrics are expressed in the units of the forecasted variable. The other three are expresses as percentages.

       

      1. Scale-dependent error metrics

      • Mean Absolute Error (MAE) aka Mean Absolute Deviation (MAD)
      • Median Absolute Error (MdAE)
      • Root Mean Square Error (RMSE)
      • These metrics express the error in the original units of the data.
        • Ex: units, cases, barrels, kilograms, dollars, liters, etc.
      • Since forecasts can be too high or too low, the signs of the errors will be either positive or negative, allowing for unwanted cancellations.
        • Ex: You don’t want errors of +50 and -50 to cancel and show “no error”.
      • To deal with the cancellation problem, these metrics take away negative signs by either squaring or using absolute value.

       

      2. Percentage error metric

      • Mean Absolute Percentage Error (MAPE)
      • This metric expresses the size of the error as a percentage of the actual value of the forecasted variable.
      • The advantage of this approach is that it immediately makes clear whether the error is a big deal or not.
      • Ex: Suppose the MAE is 100 units. Is a typical error of 100 units horrible? ok? great?
      • The answer depends on the size of the variable being forecasted. If the actual value is 100, then a MAE = 100 is as big as the thing being forecasted. But if the actual value is 10,000, then a MAE = 100 shows great accuracy, since the MAPE is only 1% of the actual.

       

      3. Relative error metric

      • Median Relative Absolute Error (MdRAE)
      • Relative to what? To a benchmark forecast.
      • What benchmark? Usually, the “naïve” forecast.
      • What is the naïve forecast? Next forecast value = last actual value.
      • Why use the naïve forecast? Because if you can’t beat that, you are in tough shape.

       

      4. Scale-Free error metric

      • Median Relative Scaled Error (MdRSE)
      • This metric expresses the absolute forecast error as a percentage of the natural level of randomness (volatility) in the data.
      • The volatility is measured by the average size of the change in the forecasted variable from one time period to the next.
        • (This is the same as the error made by the naïve forecast.)
      • How does this metric differ from the MdRAE above?
        • They do both use the naïve forecast, but this metric uses errors in forecasting the demand history, while the MdRAE uses errors in forecasting future values.
        • This matters because there are usually many more history values than there are forecasts.
        • In turn, that matters because this metric would “blow up” if all the data were zero, which is less likely when using the demand history.

       

      Intermittent Demand Planning and Parts Forecasting

       

      The special problem of intermittent demand

      • “Intermittent” demand has many zero demands mixed in with random non-zero demands.
      • MAPE gets ruined when errors are divided by zero.
      • MdRAE can also get ruined.
      • MdSAE is less likely to get ruined.

       

      Recap and remarks

      • Forecast metrics are necessary aids for monitoring and improving forecast accuracy.
      • There are two major classes of metrics: absolute and relative.
      • Absolute measures (MAE, MdAE, RMSE) are natural choices when assessing forecasts of one item.
      • Relative measures (MAPE, MdRAE, MdSAE) are useful when comparing accuracy across items or between alternative forecasts of the same item or assessing accuracy relative to the natural variability of an item.
      • Intermittent demand presents divide-by-zero problems which favor MdSAE over MAPE.
      • When assessing forecasts of multiple items, it often makes sense to use weighted averages, weighting items differently by volume or revenue.
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        Forecast error can be consequential

        Consider one item of many

        • Product X costs $100 to make and nets $50 profit per unit.
        • Sales of Product X will turn out to be 1,000/month over the next 12 months.
        • Consider one item of many

        What is the cost of forecast error?

        • If the forecast is 10% high, end the year with $120,000 of excess inventory.
        • 100 extra/month x 12 months x $100/unit
        • If the forecast is 10% low, miss out on $60,000 of profit.
        • 100 too few/month x 12 months x $50/unit

         

        Three mistakes to avoid

        1. Ignoring error.

        • Unprofessional, dereliction of duty.
        • Wishing will not make it so.
        • Treat accuracy assessment as data science, not a blame game.

        2. Tolerating more error than necessary.

        • Statistical forecasting methods can improve accuracy at scale.
        • Improving data inputs can help.
        • Collecting and analyzing forecast error metrics can identify weak spots.

        3. Wasting time and money going too far trying to eliminate error.

        • Some product/market combinations are inherently more difficult to forecast. After a point, let them be (but be alert for new specialized forecasting methods).
        • Sometimes steps meant to reduce error can backfire (e.g., adjustment).
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