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I know there are lots of variants, I am just interested in a rough average assuming there are about 6 or 7 adverts up. I'm sure I saw a thread on this somewhere before.
From a statistical viewpoint you could say that 100,000 clicks would be a good amount of data to work with when trying to figure out the differences between the top 10 positions. In practice, we have looked at this data and found no real pattern apart from a very rough-fit increase in CTR.
We have huge anomolies with our Google data but find very clear trends on the data we collect once a user visits our sites. I would say that Google has a system that inadvertantly creates 'blips'. An example of this is an advert that rund as ave pos 4, this could be #3-5 or it could be #1-7, if it's the latter then the times it appears at #1 will skew the % upwards.
My experience shows that, in our industry a position 8 advert will get around 45% of the CTR that position 1 enjoys. Plot a line from 45% to 100% and you will have a VERY rough guide for our industry. However, we have seen very different results for client industries. It's all down to how easy it is to find the information that you advertise for.
The best approach is to analyse your own data and go from there as that is what you will know the most about, and that is where you can make calls on how much you can stand in terms of ROI. As the difference from #1 to #8 can be huge in terms of CPC, it can be a good tactice to broaden your advertising but keep bids low, however if you need to back up the ads with REAL content then you have to factor in the cost of creating that content. There are many factors that will be unique to you and your business.
No-one has an average business.
This brings up another variable (the way information is presented and the quality/amount of ad copy.
Thank you for your helpful post and the useful link.
The reason we want a rough guestimation of the overall average CTR according to position on Adwords was to use it as a starting point in a programme that will improve the performance of our very big keyword campaigns by monitoring better profit according to position.
I know it's bloody tricky with adwords as you can be in 2nd place and be paying less then the person in 4th, but it's just meant to be a guide for the programme we have written.
Because of the continual changes in bids because of the millions of factors that affect them with Adwords it's impossible for anything to be very accurate, but it's just an improvement tweak we are trying out.
It's a shame Yahoo Search/Overture is so expensive as that would be a doddle.