Fifteen questions that reveal how forecasts are computed in your company

In a recent LinkedIn post, I detailed four questions that, when answered, will reveal how forecasts are being used in your business.  In this article, we’ve listed questions you can ask that will reveal how forecasts are created.

1. When we ask users how they create forecasts, their answer will often be “we use history.” This obviously isn’t enough information, as there are different types of demand history that require different forecasting methods. If you are using historical data, then make sure to find out if you are using an averaging model, a trending model, a seasonal model, or something else to forecast.

2. Once you know the model used, ask about the parameter values of those models. The forecast output of an “average” will differ, sometimes significantly, depending on the number of periods you are averaging.  So, find out whether you are using an average of the last 3 months, 6 months, 12 months, etc.

3. If you are using trending models, ask how the model weights are set. For example, in a trending model, such as double exponential smoothing, the forecasts will differ significantly depending on how the calculations weight recent data compared to older data (higher weights put more emphasis on the recent data).

4. If you are using seasonal models, the forecast results are going to be impacted by the “level” and “trending weights” used. You should also determine whether seasonal periods are forecasted with multiplicative or additive seasonality.  (Additive seasonality says, e.g., “Add 100 units for July”, whereas multiplicative seasonality says “Multiply by 1.25 for July.”) Finally, you may not be using these types of methods at all.  Some practitioners will use a forecast method that simply averages prior periods (i.e., next June will be forecasted based on the average of the prior three Junes).

5. How do you go about choosing one model over another? Does the choice of technique depend on the type of demand data or when new demand data are available? Is this process automated? Or if a planner chooses a trend model subjectively, will that item continue to be forecasted with that model until the planner changes it again?

6. Are your forecasts “fully automatic,” so that trend and/or seasonality are detected automatically? Or are your forecasts dependent on item classifications that must be maintained by users? The latter requires more time and attention from planners to define what behavior constitutes trend, seasonality, etc.

7. What are the item classification rules used? For example, an item may be considered a trending item if demand increases by more than 5% period-over-period. An item may be considered seasonal if 70% or more of the annual demand occurs in four or fewer periods. Such rules are user-defined and often require overly broad assumptions. Sometimes they are configured when a system was originally implemented but never revised even as conditions change. It’s important to make sure any classification rules are understood and, if necessary, updated.

8. Does the forecast regenerate automatically when new data are available, or do you have to manually regenerate the forecasts?

9. Do you check for any change in forecast from one period to the next before deciding whether to use the new forecast? Or do you default to the new forecast?

10. How are forecast overrides that were made in prior planning cycles treated when a new forecast is created? Are they reused or replaced?

11. How do you incorporate forecasts made by your sales team or by your customers? Do these forecasts replace the baseline forecast, or do you use these inputs to make planner overrides to the baseline forecast?

12. Under what circumstances would you ignore the baseline forecast and use exactly what sales or customers are telling you?

13. If you rely on customer forecasts, what do you do about customers who don’t provide forecasts?

14. How do you document the effectiveness of your forecasting approach?  Most companies only measure the accuracy of the final forecast that is submitted to the ERP system, if they measure anything. But they don’t assess alternative predictions that might have been used. It is important to compare what you are doing to benchmarks. For example, do the methods you are using outperform a naïve forecast (i.e., “tomorrow equals today,” which requires no thought), or what you saw last year, or the average of the last 12 months.  Benchmarking your baseline forecast insures you are squeezing as much accuracy as possible out of the data.

15. Do you measure whether overrides from sales, customers, and planners are making the forecast better or worse? This is just as important as measuring whether your statistical approaches are outperforming the naïve method.  If you don’t know whether overrides are helping or hurting, the business can’t get better at forecasting – you need to know which steps are adding value so that you can do more of those and get even better. If you aren’t documenting forecast accuracy and conducting “forecast value add” analysis, then you aren’t able to properly assess whether the forecasts being produced are the best you could make.  You’ll miss opportunities to improve the process, increase accuracy, and educate the business on what type of forecast error is to be expected.



How to interpret and manipulate forecast results with different forecast methods

Smart IP&O is powered by the SmartForecasts® forecasting engine that automatically selects the most appropriate method for each item.  Smart Forecast methods are listed below:

  • Simple Moving Average and Single Exponential Smoothing for flat, noisy data
  • Linear Moving Average and Double Exponential Smoothing for trending data
  • Winters Additive and Winters Multiplicative for seasonal and seasonal & trending data.

This blog explains how each model works using time plots of historical and forecast data.  It outlines how to go about choosing which model to use.   The examples below show the same history, in red, forecasted with each method, in dark green, compared to the Smart-chosen winning method, in light green.


If you want to force (or prevent) seasonality to show in the forecast, then choose Winters models.  Both methods require 2 full years of history.

`Winter’s multiplicative will determine the size of the peaks or valleys of seasonal effects based on a percentage difference from a trending average volume.  It is not a good fit for very low volume items due to division by zero when determining that percentage. Note in the image below that the large percentage drop in seasonal demand in the history is being projected to continue over the forecast horizon making it look like there isn’t any seasonal demand despite using a seasonal method.


Winter’s multiplicative Forecasting method software

Statistical forecast produced with Winter’s multiplicative method. 


Winter’s additive will determine the size of the peaks or valleys of seasonal effects based on a unit difference from the average volume.  It is not a good fit if there’s significant trend to the data.  Note in the image below that seasonality is now being forecasted based on the average unit change in seasonality. So, the forecast still clearly reflects the seasonal pattern despite the down trend in both the level and seasonal peaks/valleys.

Winter’s additive Forecasting method software

Statistical forecast produced with Winter’s additive method.



If you want to force (or prevent) trend up or down to show in the forecast, then restrict the chosen methods to (or remove the methods of) Linear Moving Average and Double Exponential Smoothing.

 Double exponential smoothing will pick up on a long-term trend.  It is not a good fit if there are few historical data points.

Double exponential smoothing Forecasting method software

Statistical forecast produced with Double Exponential Smoothing


Linear moving average will pick up on nearer term trends.  It is not a good fit for highly volatile data

Linear moving average Forecasting method software


Non-Trending and Non-Seasonal Data
If you want to force (or prevent) an average from showing in the forecast, then restrict the chosen methods to (or remove the methods of) Simple Moving Average and Single Exponential Smoothing.

Single exponential smoothing will weigh the most recent data more heavily and produce a flat-line forecast.  It is not a good fit for trending or seasonal data.

Single exponential smoothing Forecasting method software

Statistical forecast using Single Exponential Smoothing

Simple moving average will find an average for each period, sometimes appearing to wiggle, and better for longer-term averaging.  It is not a good fit for trending or seasonal data.

Simple moving average Forecasting method software

Statistical forecast using Simple Moving Average




What to do when a statistical forecast doesn’t make sense

Sometimes a statistical forecast just doesn’t make sense.  Every forecaster has been there.  They may double-check that the data was input correctly or review the model settings but are still left scratching their head over why the forecast looks very unlike the demand history.   When the occasional forecast doesn’t make sense, it can erode confidence in the entire statistical forecasting process.

This blog will help a layman understand what the Smart statistical models are and how they are chosen automatically.  It will address how that choice sometimes fails, how you can know if it did, and what you can do to ensure that the forecasts can always be justified.  It’s important to know to expect, and how to catch the exceptions so you can rely on your forecasting system.


How methods are chosen automatically

The criteria to automatically choose one statistical method out of a set is based on which method came closest to correctly predicting held-out history.  Earlier history is passed to each method and the result is compared to actuals to find the one that came closest overall.  That automatically chosen method is then fed all the history to produce the forecast. Check out this blog to learn more about the model selection

For most time series, this process can capture trends, seasonality, and average volume accurately. But sometimes a chosen method comes mathematically closest to predicting the held-out history but doesn’t project it forward in a way that makes sense.  That means the system selected method isn’t best and for some “hard to forecast”


Hard to forecast items

Hard to forecast items may have large, unpredictable spikes in demand, or typically no demand but random irregular blips, or unusual recent activity.  Noise in the data sometimes randomly wanders up or down, and the automated best-pick method might forecast a runaway trend or a grind into zero.  It will do worse than common sense and in a small percentage of any reasonably varied group of items.  So, you will need to identify these cases and respond by overriding the forecast or changing the forecast inputs.


How to find the exceptions

Best practice is to filter or sort the forecasted items to identify those where the sum of the forecast over the next year is significantly different than the corresponding history last year.  The forecast sum may be much lower than the history or vice versa.  Use supplied metrics to identify these items; then you can choose to apply overrides to the forecast or modify the forecast settings.


How to fix the exceptions

Often when the forecast seems odd, an averaging method, like Single Exponential Smoothing or even a simple average using Freestyle, will produce a more reasonable forecast.  If trend is possibly valid, you can remove only seasonal methods to avoid a falsely seasonal result.  Or do the opposite and use only seasonal methods if seasonality is expected but wasn’t projected in the default forecast.  You can use the what-if features to create any number of forecasts, evaluate & compare, and continue to fine tune the settings until you are comfortable with the forecast.

Cleaning the history, with or without changing the automatic method selection, is also effective at producing reasonable forecasts. You can embed forecast parameters to reduce the amount of history used to forecast those items or the number of periods passed into the algorithm so earlier, outdated history is no longer considered.  You can edit spikes or drops in the demand history that are known anomalies so they don’t influence the outcome.  You can also work with the Smart team to implement automatic outlier detection and removal so that data prior to being forecasted is already cleansed of these anomalies.

If the demand is truly intermittent, it is going to be nearly impossible to forecast “accurately” per period. If a level-loading average is not acceptable, handling the item by setting inventory policy with a lead time forecast can be effective.  Alternatively, you may choose to use “same as last year” models which while not prone to accuracy will be generally accepted by the business given the alternatives forecasts.

Finally, if the item was introduced so recently that the algorithms do not have enough input to accurately forecast, a simple average or manual forecast may be best.  You can identify new items by filtering on the number of historical periods.


Manual selection of methods

Once you have identified rows where the forecast doesn’t make sense to the human eye, you can choose a smaller subset of all methods to allow into the forecast run and compare to history.  Smart will allow you to use a restricted set of methods just for one forecast run or embed the restricted set to use for all forecast runs going forward. Different methods will project the history into the future in different ways.  Having a sense of how each works will help you choose which to allow.


Rely on your forecasting tool

The more you use Smart period over period to embed your decisions about how to forecast and what historical data to consider, the less often you will face exceptions as described in this blog.  Entering forecast parameters is a manageable task when starting with critical or high impact items.  Even if you don’t embed any manual decisions on forecast methods, the forecast re-runs every period with new data. So, an item with an odd result today can become easily forecastable in time.



Spare Parts Planning Isn’t as Hard as You Think

When managing service parts, you don’t know what will break and when because part failures are random and sudden. As a result, demand patterns are most often extremely intermittent and lack significant trend or seasonal structure. The number of part-by-location combinations is often in the hundreds of thousands, so it’s not feasible to manually review demand for individual parts. Nevertheless, it is much more straightforward to implement a planning and forecasting system to support spare parts planning than you might think.

This conclusion is informed by hundreds of software implementations we’ve directed over the years. Customers managing spare parts and service parts (the latter for internal consumption/MRO), and to a lesser degree aftermarket parts (for resale to installed bases), have consistently implemented our parts planning software faster than their peers in manufacturing and distribution.

The primary reason is the role in manufacturing and distribution of business knowledge about what might happen in the future. In a traditional B2B manufacturing and distribution environment, there are customers and sales and marketing teams selling to those customers. There are sales goals, revenue expectations, and budgets. This means there is a lot of business knowledge about what will be purchased, what will be promoted, whose opinions need to be accounted for. A complex planning loop is required. In contrast, when managing spare parts, you have a maintenance team that fixes equipment when it breaks. Though there are often maintenance schedules for guidance, what is needed beyond a standard list of consumable parts is often unknown until a maintenance person is on-site. In other words, there just isn’t the same sort of business knowledge available to parts planners when making stocking decisions.

Yes, that is a disadvantage, but it also has an upside: there is no need to produce a period-by-period consensus demand forecast with all the work that requires. When planning spare parts, you can usually skip many steps required for a typical manufacturer, distributor, or retailer. These skippable steps include:  

  1. Building forecasts at different levels of the business, such as product family or region.
  2. Sharing the demand forecast with sales, marketing, and customers.
  3. Reviewing forecast overrides from sales, marketing, and customers.
  4. Agreeing on a consensus forecast that combines statistics and business knowledge.
  5. Measuring “forecast value add” to determine if overrides make the forecast more accurate.
  6. Adjusting the demand forecast for known future promotions.
  7. Accounting for cannibalization (i.e., if I sell more of product A, I’ll sell less of product B).

Freed from a consensus-building process, spare parts planners and inventory managers can rely directly on their software to predict usage and the required stocking policies. If they have access to a field-proven solution that addresses intermittent demand, they can quickly “go live” with more accurate demand forecasts and estimates of reorder points, safety stocks, and order suggestions.  Their attention can be focused on getting accurate usage and supplier lead time data. The “political” part of the job can be limited to obtaining organization consensus on service level targets and inventory budgets.

Spare Parts Planning Software solutions

Smart IP&O’s service parts forecasting software uses a unique empirical probabilistic forecasting approach that is engineered for intermittent demand. For consumable spare parts, our patented and APICS award winning method rapidly generates tens of thousands of demand scenarios without relying on the assumptions about the nature of demand distributions implicit in traditional forecasting methods. The result is highly accurate estimates of safety stock, reorder points, and service levels, which leads to higher service levels and lower inventory costs. For repairable spare parts, Smart’s Repair and Return Module accurately simulates the processes of part breakdown and repair. It predicts downtime, service levels, and inventory costs associated with the current rotating spare parts pool. Planners will know how many spares to stock to achieve short- and long-term service level requirements and, in operational settings, whether to wait for repairs to be completed and returned to service or to purchase additional service spares from suppliers, avoiding unnecessary buying and equipment downtime.

Contact us to learn more how this functionality has helped our customers in the MRO, Field Service, Utility, Mining, and Public Transportation sectors to optimize their inventory. You can also download the Whitepaper here.



White Paper: What you Need to know about Forecasting and Planning Service Parts


This paper describes Smart Software’s patented methodology for forecasting demand, safety stocks, and reorder points on items such as service parts and components with intermittent demand, and provides several examples of customer success.


    The Role of Trust in the Demand Forecasting Process Part 2: What do you Trust

    “Regardless of how much effort is poured into training forecasters and developing elaborate forecast support systems, decision-makers will either modify or discard the predictions if they do not trust them.”  — Dilek Onkal, International Journal of Forecasting 38:3 (July-September 2022), p.802.

    The words quoted above grabbed my attention and prompted this post. Those of a geekly persuasion, like your blogger, are inclined to think of forecasting as a statistical problem. While that is obviously true, those of a certain age, like your blogger, understand that forecasting is also a social activity and therefore has a large human component.

    What Do You Trust?

    There is a related dimension of trust: not who do you trust but what do you trust? By this, I mean both data and software.

    Trust in Data

    Trust in data underpins trust in the forecaster using the data. Most of our customers have their data in an ERP system. This data must be understood as a key corporate asset. For the data to be trustworthy, it must have the “three C’s”, i.e., it must be correct, complete, and current.

    Correctness is obviously fundamental. We once had a customer who was implementing a new, strong forecasting process, but found the results completely at odds with their sense of what was happening in the business. It turned out that several of their data streams were incorrect by a factor of two, which is a huge error. Of course, this set back the implementation process until they could identify and correct all the gross errors in their demand data.

    There is a less obvious point to be made about correctness. That is, data are random, so what you see now is not likely to be what you see next. Planning production based on the assumption that next week’s demand will be exactly the same as this week’s demand is clearly foolish, but classical formula-based forecasting models like the exponential smoothing mentioned above will project the same number throughout the forecast horizon. This is where scenario-based planning is essential for coping with the inevitable fluctuations in key variables such as customers’ demands and suppliers’ replenishment lead times.

    Completeness is the second requirement for data to be trusted. Our software ultimately gets much of its value from exposing the links between operational decisions (e.g., selecting the reorder points governing replenishment of stock) and business-related metrics like inventory costs. Yet often implementation of forecasting software is delayed because item demand information is available someplace, but holding, ordering and/or shortage costs are not.  Or, to cite another recent example, a customer was able to properly size only half their inventory of spares for reparable parts because nobody had been tracking when the other half was breaking down, meaning there was no information on mean time before failure (MTBF), meaning it was not possible to model the breakdown behavior of half the fleet of reparable spares.

    Finally, the currency of data matters. As the speed of business increases and company planning cycles drop from a quarterly or monthly tempo to a weekly or daily tempo, it becomes desirable to exploit the agility provided by overnight uploads of daily transactional data into the cloud. This allows high-frequency adjustments of forecasts and/or inventory control parameters for items that experience high volatility and sudden shifts in demand. The fresher the data, the more trustworthy the analysis.

    Trust in Demand Forecasting Software

    Even with high-quality data, forecasters must still trust the analytical software that processes the data. This trust must extend to both the software itself and to the computational environment in which it functions.

    If forecasters used on-premises software, they must rely on their own IT departments to safeguard the data and keep it available for use. If they wish instead to exploit the power of cloud-based analytics, customers must trust their confidential information to their software vendors. Professional-level software, such as ours, justifies customers’ trust through SOC 2 certification. SOC 2 certification was developed by the American Institute of CPAs and defines criteria for managing customer data based on five “trust service principles”—security, availability, processing integrity, confidentiality, and privacy.

    What about the software itself? What is needed to make it trustworthy? The main criteria here are the correctness of algorithms and functional reliability. If the vendor has a professional program development process, there will be little chance that the software ends up computing the wrong numbers because of a programming error. And if the vendor has a rigorous quality assurance process, there will be little chance that the software will crash just when the forecaster is on deadline or must deal with a pop-up analysis for a special situation.


    To be useful, forecasters and their forecasts must be trusted by decision-makers. That trust depends on characteristics of forecasters and their processes and communication. It also depends on the quality of the data and software used in creating the forecasts.


    Read the 1st part of this Blog “Who do you Trust” here: