calculating CTR for a page

[sallam]

Greetings

To know the CTR a web page generates, do I add the CTR of each adsense ad in that page?
Example: I used to have one link unit that gave an average CTR of 3.8%
and one banner that gave 9%.
Now I've added a skyscraper that now gives 5%, but it affected the banner, which now gives only 5.3% and the link unit was also reduced to 3%
But if I add the total now and the total then, I find an increase of (13.3 - 12.8 =) 0.5%

Am I right in that way of adding ctr? this is crucial to help me decide whether to keep the third ad or turn back to only 2.

[joelgreen]

You cannot just add CTRs. To know the CTR a web page generates you have to divide all clicks page on the page by page impressions (for a given period).

CTR = Clicks / Impressions * 100%

[sallam]

same thing I guess.
Lets say page impression is 1000, ad 1 got 10 clicks (ctr=1%) and ad 2 got 90 clicks (ctr=9%).
Now using your method:
10+90=100
1000/100=10%

and using mine:
1+9=10%

Same result, only simpler.
But I need to verify this is the proper way to judge ads on a page.