Before I start, I was hoping to bring you some figures on the number of users who own a scroll-mouse, or at least some recent sales statistics. While I haven't yet found that data, I can say that I did find that the scroll-mouse (especially optical) are among the most popular because of the ease it offers in scrolling documents.

With that in mind, the "Adsense Scroll Mouse Theory" goes something like this:

People scroll a webpage in mainly two different ways: A) They are browsing quickly and looking for something to catch their eye, or B) They are reading a document where their scrolling is much slower and paced. When a user scrolls the page, there are natural breaks which occur due to limited finger movement. Based on the two kinds of scrolling, finger movements and the fact that eye-level content gets the most visibility, an estimate can be made (in pixels) which determines where the majority of these breaks occur at eye-level.

In this experiment, I recorded results from 10 different scroll-mouse users to measure their scrolls in pixels, based on two different scrolling habits.

A) Fast browsing - the average mouse scroll is 600-700 pixels vertically, meaning when the user pauses, their eye-level falls within the 600-700 pixels range on the page (from the top of the document, not the browser). They scroll and stop, scroll and stop. They have no choice.

B) Slow browsing - the average slow mouse scroll is 400-460 pixels vertically (scrolling in smaller chunks while reading), meaning that they pause at eye level every 400-460 pixels vertically. As they read a section, they usually start to scroll bringing more content up into their immediate eye-level.

Considering these two key mouse scrolling behaviours, it would make sense to place ads exactly at these points, or use an overall average to concentrate ads within every 400-700 vertical pixels on the page. The best fit would be a 336x280 ad block as it catches both ends of the slow & fast scroll.

You might be wondering about people using a browser's scrollbar instead of a scroll-mouse. With a browser's scrollbar, fast scrolling habits either stopped at around the 2000 pixel mark (for very long pages equal to just slightly more than half-way down a page) or the bottom of the page. For slow scrolling habits, the pauses occurred at eye-level about every 400-460 pixels, just as with a scroll-mouse.

The consistency in slow mouse & slow browser scrolling habits would leave you to assume that Adsense ads placed around the 400-460 mark might be something worth testing on your own website.

But consider this too: Initial eye-level on a webpage is somewhere around the 200-231 pixel mark. You could try placing ads at this point, the 400-460 mark, and then at the 600-700 mark. This way you are getting immediate visibility with the top ads first presented when the page loads, caching slow readers as they scroll, and catching fast scrollers too. The only group you'd have a problem slowing down are the ones using browser scrollbars. Images used next to Adsense ads can help catch their attention, especially if the page has a lot of text.

Now it's very likely someone else has already thought of this and it's already been discussed to death. I haven't found anything related to it. Everything here is really just theory, but something I plan to test myself using live test subjects (it sounds cooler than 'search users') and will post results as they come.

I would love to hear if you tested this out and what your results were.