In Part 1 and Part 2 of our Lemonade Stand series, we demonstrated how to add a trend line to a chart in FileMaker 12. In this part, we’re going to look at how to create a seasonal index from a set of historical data and then use that index to create a forecast based on both the trend and seasonality.
Calculating the seasonal index
To find the seasonal index for a given month we simply divide the average historical sales for the month by the average monthly sales for all months. The period for a seasonal index does not have to be a month. It could be a quarter, a week, a day of the week, or even an hour of the day. The same technique applies.
Let's look at an example for our lemonade stand. The following chart shows the January sales for each of the past three years.