Referencing the Last Six Items in a Formula

Darryl needs to find the average of the last six entries in a column. The number of items in the column can vary over time as more information is added, but he always wants the average of those last six items.

There are a number of different formulas you can use, probably too many to go over here in detail. With that in mind, it is instructive to look at two particular formulas. The one you choose to use will depend on the characteristics of the data in the column. If there are no blank cells in the column, then finding the average can be done with a relatively simple formula:

=AVERAGE(OFFSET(A1,COUNT(A:A)-6,0,6,1))

This formula uses the OFFSET function to calculate the proper cells to examine, at the bottom of the column. This formula won't work if there are blank cells in the column. In that case you will need to use a formula that examines the contents of each cell and determines, as part of the calculation process, whether it is blank or not. Array formulas or regular formulas using array functions are great for this purpose. The following example uses the SUMPRODUCT function to accomplish the task:

=SUMPRODUCT((A1:A30*((MAX(ROW(A1:A30)*(A1:A30<>""))-ROW(A1:A30))<6))/6)

This formula assumes that the cells to be evaluated are in the range of A1:A30; it doesn't matter if there are blank cells in this range. The ROW functions are used to create arrays that determine if the individual cells contain values or not. Only those rows containing values end up being counted, and those are divided by 6 and summed, providing the desired average.