Q:

Easier way to calcuate average change?

I have a product that is getting sent out to the field as it is manufactured and used daily, the use rate changes from day to day based on how the job is going. 
I am trying to calculate the average amount of this product used on a daily basis. I need to subtract the amount used from one day to the next, and average them out. 
Is there an easier way to do this than =average((a2-a1),(a3-a2),etc) ?

A:

If column A includes the cumulative usage at the end of every day, you don't need to use the AVERAGE function at all. You can calculate the average by subtracting the first value from the last value and dividing by the number of days.
Enter this formula in B2 (or any empty cell in row 2)
=(A2-A$1)/ROWS(A$2:A2)

And copy it down to the other cells in the same column. B2 will now show the average daily usage after one day, C2 will show the average daily usage over the first two days, and so on.

