Average of an Average

[PWD]

Good morning, just a mathematical query. I'm exporting a set of average lengths of stay per hospital ward from Business Objects. (How lucky am I???) Is it going to be mathematically correct just to obtain an average of those average figures to give an overall figure for the hospital?

I was going to output all of the individual lengths of stay and calculate the average to check but Business Objects has just gone stupid on me and now refuses to use the UK English time format now I've re-opened the report!!

Many thanks,
D€$

 

[PWD]

Short answer, "No." The original Business Objects calculation will work because there are some patients who have a length of stay less than a day, the Excel calculation would need to include any wards where there was a genuine 0 length of stay but exclude any (closed) wards in the worksheet for which no data is returned. Oh well. I've created a new sheet in BO that shows the average on individuals' stays.

Many thanks,
D€$

 

[jonsi]

Mathematically it would be wholly wrong

try this

7 whole numbers summed that equal 25 :3,6,3,3,5,2,3 then look at the average 25/7 = 3.571429
5 whole numbers summed that equal 25 :5,5,4,6,5 then look at the average 25/5 = 5

Average those two numbers summed 8.571429/2 = 4.2857

Now divide 50 (25 + 25) by 12 (the count of the numbers above) = 4.166667

Which 'average' are you going to choose as your correct one? See how far out you could be even with simple calculations?

Never average an average

Note: the average number of legs on a dog is less than 4!

hwyl
Jonsi B-)
"If an apple a day keeps the Doctor away ...why don't Daleks live in Orchards?"

 

[PWD]

Yeah Jonsi, I thought I would be on a sticky wicket. Thanks for the very useful illustration!

Many thanks,
D€$

 

[combo]

For each average you need also a number of cases insluded, having this [tt]TotalAverage = SUMPRODUCT(Averages, NoOfCasesInAverages) / SUM(NoOfCasesInAverages)[/tt]

combo