IN THIS CHAPTER
Knowing the different methods of forecasting
Arranging your data in an order Excel can use
Getting acquainted with the Analysis ToolPak
Going it alone
A sales forecast is like a weather forecast: Itâs an educated guess at what the future will bring. You can forecast all sorts of things â poppy-seed sales, stock market futures, the weather â in all sorts of ways: You can make your own best guess; you can compile and composite other peopleâs guesses; or you can forecast on the basis of wishful thinking.
Unfortunately, none of these options is truly acceptable. If you want to make better forecasts, you need to take advantage of some better options. And there are different ways to forecast, ways that have proven their accuracy over and over. They take a little more time to prepare than guessing does, but in the long run Iâve spent more time explaining bad guesses than doing the forecasts right in the first place.
Microsoft Excel was originally developed as a spreadsheet application, suited to figuring payment amounts, interest rates, account balances, and so on. But as Microsoft added more and more functions â for example, AVERAGE and TREND and inventory-management stuff â Excel became more of a multipurpose analyst than a single-purpose calculator.
Excel has the tools you need to make forecasts, whether you want to prepare something quick and dirty (and who doesnât from time to time?) or something sophisticated enough for a boardroom presentation.
The tools are there. You just need to know which tool to choose for which situation and, of course, how to use it. You need to know how to arrange data for the tool. And you need to know how to interpret what the tool tells you â whether that toolâs a basic one or something more advanced.
Understanding Excel Forecasts
If you want to forecast the future â next quarterâs sales, for example â you need to get a handle on whatâs happened in the past. So you always start with whatâs called a baseline (that is, past history â how many poppy seeds a company sold during each of the last ten years, where the market futures wound up each of the last 12 months, what the daily high temperature was year-to-date).
Unless youâre going to just roll the dice and make a guess, you need a baseline for a forecast. Today follows yesterday. What happens tomorrow generally follows the pattern of what happened today, last week, last month, last quarter, last year. If you look at whatâs already happened, youâre taking a solid step toward forecasting whatâs going to happen next. (Part 1 of this book talks about forecast baselines and why they work.)
An Excel forecast isnât any different from forecasts you make with a specialized forecasting program. But Excel is particularly useful for making sales forecasts, for a variety of reasons:
- You often have sales history recorded in an Excel worksheet. When you already keep your sales history in Excel, basing your forecast on the existing sales history is easy â youâve already got your hands on it.
- Excelâs charting features make it much easier to visualize whatâs going on in your sales history and how that history defines your forecasts.
- Excel has tools (found in whatâs called the Data Analysis add-in) that make generating forecasts easier. You still have to know what youâre doing and what the tools are doing â you donât want to just jam the numbers through some analysis tool and take the result at face value, without understanding what the toolâs up to. But thatâs what this book is here for.
- You can take more control over how the forecast is created by skipping the Data Analysis add-inâs forecasting tools and entering the formulas yourself. As you get more experience with forecasting, youâll probably find yourself doing that more and more.
You can choose from several different forecasting methods, and itâs here that judgment begins. The three most frequently used methods, in no special order, are moving averages, exponential smoothing, and regression.
Method #1: Moving averages
Moving averages may be your best choice if you have no source of information other than sales history â but you do need to know your baseline sales history. Later in this chapter, I show you more of the logic behind using moving averages. The underlying idea is that market forces push your sales up or down. By averaging your sales results from month to month, quarter to quarter, or year to year, you can get a better idea of the longer-term trend thatâs influencing your sales results.
For example, you find the average sales results of the last three months of last year â October, November, and December. Then you find the average of the next three-month period â November, December, and January (and then December, January, and February; and so on). Now youâre getting an idea of the general direction that your sales are taking. The averaging process evens out the bumps you get from discouraging economic news or temporary boomlets.
Method #2: Exponential smoothing
Exponential smoothing is closely related to moving averages. Just as with moving averages, exponential smoothing uses past history to forecast the future. You use what happened last week, last month, and last year to forecast what will happen next week, next month, or next year.
The difference is that when you use smoothing, you take into account how bad your previous forecast was â that is, you admit that the forecast was a little screwed up. (Get used to that â it happens.) The nice thing about exponential smoothing is that you take the error in your last forecast and use that error, so you hope, to improve your next forecast.
If your last forecast was too low, exponential smoothing kicks your next forecast up. If your last forecast was too high, exponential smoothing kicks the next one down.
The basic idea is that exponential smoothing corrects your next forecast in a way that would have made your prior forecast a better one. Thatâs a good idea, and it usually works well.
Method #3: Regression
When you use regression to make a forecast, youâre relying on one variable to predict another. For example, when the Federal Reserve raises short-term interest rates, you might rely on that variable to forecast whatâs going to happen to bond prices or the cost of mortgages. In contrast to moving averages or exponential smoothing, regression relies on a different variable to tell you whatâs likely to happen next â something other than your own sales history.