# How to compute a weighted average?

4 transactions take 10 days each to complete 2 transactions take 8 days each to complete 9 transactions take 11 days each to complete 1 transaction takes 40 days to complete

How does one compute the weighted average time to complete a transaction?

Thanks!

-
Well how do you wanna weight the average? What did you try? –  Dominic Michaelis Mar 14 '13 at 21:21
add comment

## 3 Answers

Just average together the list [10, 10, 10, 10, 8, 8, 11, 11, 11, 11, 11, 11, 11, 11, 11, 40].

-
add comment

It's (total time to complete all transactions)/(number of transactions)

$$\frac{4*10 + 2*8 + 9*11 + 1*40}{4 + 2 + 9 + 1}$$

-
add comment

The answers seem like just simple averages, not weighted averages. I dont want the 1 transaction that takes 40 days to have much of an impact on the result, because the other 15 transactions fall in the 8 to 11 day range. The 40 day transaction is sort of an aberration, and I want it to carry less weight in computing the result. Does that make sense?

-
What is done (and I'm far from an expert here!) is to see if a data point is a outlier (essentially by seeing if it is "too far" from the others) and leaving it out if so. Search for that (or ask for that case here). –  vonbrand Mar 14 '13 at 23:47
add comment