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Please Note: This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: Averaging the Last Numbers in a Column.
Emma has a list of numbers in a worksheet (let's say in column A) that are added to on a weekly basis. She needs to calculate the average of the last 12 numbers in the column. She wonders how she can do this and have the average always reflect the last 12 numbers, even when she keeps adding numbers each week.
Assuming that there are no gaps in your range of numbers, you can calculate the average of the last 12 numbers with this formula:
This formula should, of course, be placed in some cell that is not in column A. It uses the COUNTA function to figure out how many cells contain something in column A. If there are 100 cells in use in column A, this means that you end up with a formula being evaluated in this way:
Of course, 100 minus 12 is 88, and this number is used as an offset from the starting cell (A2) to say that the range to be averaged should start at A89 and extend down 12 cells. That means that the average ends up being for the range A89:A100. As more numbers are added at the bottom of column A, the formula always reflects the last 12 numbers.
The formula will return an error if column A has fewer than 12 rows worth of data in it. To accommodate that possibility, you may want to alter the formula just a bit:
Instead of using a hard-and-fast value of 12 rows, the MIN function (in two places) returns the minimum of either the actual number of rows or 12. So, if your worksheet only has numbers in cells A1:A5, the MIN function would ensure that the formula only averaged those 5 values.
ExcelTips is your source for cost-effective Microsoft Excel training. This tip (10277) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: Averaging the Last Numbers in a Column.
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