Depends on language and reading direction. Also depends on whether your ad is an interruption or aside. If you are looking for a "sweet spot" join the club as it appears to make little difference either way.

Layout also plays a part as the eye/brain does break pages into sections (coding that would be a div) if done that way.

Call to action often means color as well as text/image content.

Size of device and viewport is also a major factor.

Yes, there are studies, but (IMO) these are a variety of crap shoot.

There's blended ads and obvious, which do better? Again, depends on the site content and the user.

What does seem to work is logical placement for the SITE and the USER, knowing that ad blindness is also part of the scheme of things.

What is more important is to corral any expectations above 10% interest and 10% conversion of that 10% (hence the 1000 metric used by most ad services)

Ads are seen, largely ignored, but if clicked that number is very small.

Personally I float ads right as I do create my websites for content and in a left/right reading world want my content front. HOWEVER, for inbody ads with three on a page it is usually:

float right
float left
float right

to break up the text column and urge the eye to see ALL of the screen, not just the content.

I have just recently conducted an experiment on one of my sites "nearly" about this subject. It was not float left or right but a comparison between central ad placement on the page (no content on either side) versus the ad floated left (with content on the right).

The ad floated left is the winner so far. A couple of months more data is needed before I make any final conclusions.

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