# Issues with rounding

1. ## Issues with rounding

I'm not sure if this is a excel question or a maths one.

I have three samples:

Population Number Rate Per 1000 Population
1000 150 150.000
2000 160 80.000
3000 170 56.667

In excel I've calculated the rate per 1000 by dividing the "number" by the "population" divided by 1000.

Anyway, I get the rates above. The problem comes with the average for the group. If I used Excel and average all three rate cells I get an overall average of 95.56.

HOWEVER if I sum the total "population" and the total "number" and divide using the same formula as above (i.e. dividing the "number" by the "population" divided by 1000) then I get 80.00

Is this Excel rounding numbers in the background? It all seems a bit wierd to me as they should be the same figure surely???

Many thanks  Register To Reply

2. ## Re: Issues with rounding

To arrive at the same results, you need to take the population-weighted average of the averages. Take a more extreme example: ``Please Login or Register  to view this content.``
It should not surprise you that the average of the averages is not the same as percent calculated for the total.  Register To Reply

3. ## Re: Issues with rounding

As shg said, you cannot take an average of averages, it is not mathematically correct. the only true average in your situation is an average calculated on the total values  Register To Reply