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It depends entirely on your audience, and topic. If you have a website where your readers are determined to get through the entire piece (ie/ a movie review or famous columnist), then the bottom is the best place. Otherwise, placing it near or at the top will suck away your readers if an ad comes up that's more relevant to what they're looking for.
Nothing beats testing it with different channels on your own site. Although be warned that its not easy to compare two pages or two sections of your website: you might be better off changing it on the whole site for a day once you've established what the normal minimum and maximums are for that day.
Thing about AdSense is that its highly variable. Beyond "weekends are slow", or "days are more popular than nights", it can be difficult to determine constant statistics.
Hope that helps,
We also had good results with a leaderboard about 400 pixels down from the top.
Not everyone will reach the bottom of the page.
As for me using top or bottom. Have not really analysed it - some we go top, others skyscraper etc. We do use channels for a certain amount of info but as the pages themselves differ in content it would be wrong for me to draw any serious conclusions.
a skyscraper 160 x 600 in the left hand margin
That's interesting. I have mine mostly in the right hand margin, as I didnt want people to confuse it for navigation...
If i swapped the navigation and adsense around (left and right columns) I assume my CTR would go up, but would the sacrifice in usability on the site be worth it?
Especially at times when Google couldn't fully fill the skyscraber with text links.
So I changed them to Large Rectangles and moved them to the bottom of the pages.
However, I didn't see any significant difference in the results.