Average of an Average

[willow1985]

I suppose this is more of a math question but I also want to know if I am calculating this right in excel / would be getting the correct numbers.

I want to get the average turn around time (time between 2 dates) for the last 30 days.

My data is as follows:

[TABLE="width: 624"]
<tbody>[TR]
[TD]Date 1[/TD]
[TD]Date 2[/TD]
[TD]Difference between dates[/TD]
[TD][/TD]
[TD][/TD]
[TD]Sum[/TD]
[TD]Count[/TD]
[TD]Average[/TD]
[TD]Average for the Month: 3.66[/TD]
[/TR]
[TR]
[TD]30-Oct[/TD]
[TD]01-Nov[/TD]
[TD]2[/TD]
[TD][/TD]
[TD]30-Oct[/TD]
[TD]8[/TD]
[TD]2[/TD]
[TD]4[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]30-Oct[/TD]
[TD]05-Nov[/TD]
[TD]6[/TD]
[TD][/TD]
[TD]29-Oct[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]25-Oct[/TD]
[TD]02-Nov[/TD]
[TD]8[/TD]
[TD][/TD]
[TD]28-Oct[/TD]
[TD]2[/TD]
[TD]1[/TD]
[TD]2[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]28-Oct[/TD]
[TD]30-Oct[/TD]
[TD]2[/TD]
[TD][/TD]
[TD]27-Oct[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]25-Oct[/TD]
[TD]30-Oct[/TD]
[TD]5[/TD]
[TD][/TD]
[TD]26-Oct[/TD]
[TD]0[/TD]
[TD]0[/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[TD]25-Oct[/TD]
[TD]5[/TD]
[TD]1[/TD]
[TD]5[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

(Note: There are multiple years so my formula only looks at 2019)

I have a Sum column and a Count Column and then an Average Column

I Sum everything on Oct 30: 8
I count everything for Oct 30: 2
I calculate the average for Oct 30: 4

Then at the end I average the average column for the last 30 days.

Would this give me the correct numbers for the average for the month?

This is probably a dumb question and something I should know but I seem to have a road block...

Thank you to anyone who can help me clear it.

Carla

 

[mikerickson]

Taking the average of averages gives you a garbage number.

To take the grand average, you need to divide the grand total sum by the grand total count.

 

[Eric W]

An average of averages is usually not a good idea. It only works if the count for each average is the same, which is not the case in your example. You need to add up the individual sums (8+2+5) and divide by the sum of the counts (2+1+1), so you would want 15/4 = 3.75.

 

[willow1985]

Thank you very much. I knew I was doing it wrong but could not remember the correct way for the life of me.

I did the following formula on the sum and count columns: =ROUND(SUM(E2:E32)/SUM(F2:F32),0)

Thank you guys!

Carla

 

[Eric W]

That's a much better formula.

Glad we could help.