I came across a very interesting piece of information today by accident. The Yahoo PPC impression numbers are hugely off. Overseeing two accounts in the same space - I see that they sit 1 and 2 and get around the same impressions - so they act as checks when I need to find reasons things are happening.
One site started advertising on MSN using the Featured listings that generally occupy the top 2 spots (I have been told it goes to three, but think it just roates who is in the top 2 spots).
Now since MSN and Yahoo are both appearing on the page, Yahoo does not duplicate the results. But that does not stop them from adding them into the impressions that they then use to calculate the CTR.
While they may think it does not matter because they do not use the CTR as Google does to influence price or position, what they are doing by disseminating this incorrect information is allowing clients to make decisions with very wrong numbers.
There at least should be a caveat at the top of the Overture account: If you are using MSN Featured listings our numbers will be inaccurate.
What is going to happen when MSN starts its PPC and Yahoo is the filler until they sell enough spots? Will it be then that people start to see this problem.
Just a warning folks.