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How to position ads on your website
Place an ad on the top fold.
Consulting firm nielsen norman group says web users spend 80% of their time looking at information above the middle of page fold. It can be placed on top of the text and will be the first ad the user will see once the page is loaded.
The best measurement for an ad in the top fold is 728x90. 728x90 fits perfectly on top of a browser page and is hard to ignore.