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I have started sprinkling some single link ad units around in some of my longer text articles. They blend in well and don't ruin the design.
However, I have a question. All things being equal, do all of these ad units pay the same per click?
For example, should I opt for a small rectangle, or a button over a half banner? Or does it not make any difference.
I'd be inclined to put a channel on each banner and see what ones work best for you. That way you can see how experiments in placement and banner type work best on your site, and dump the ones that don't work that may be dragging down your overall earnings.
Which is why I said all things being equal.
There isn't really a yes or no answer to this one is the bottom line. I know you mention single unit banners, but the same arguments apply to all banners - regardless of number of ads intended to be in them, and number of ads actually shown. The price you get depends on many factors, and whatever banners you choose will always perform differently under varying circumstances. It's all part of the adsense roller coaster that you have to get used to.
A single ad unit, or a multiple ad unit can perform vastly differently by just shifting it to a slightly different position on the page. That's why I suggest experimenting to find out what works best on your site. What works on my site may very well not work on your site, or any other site.
Basically because the "All things being equal" scenario never happens with adsense - things are NEVER equal! A click on one advertiser varies hugely depending on time of day, where the click was from, what the auction is doing. The same ad might be paying between 5c and $5 - and that's just over course of one day, in one ad block. Therefore saying what ad block generates the most overall is not possible IMHO.
What does make a difference though is ad placement, and selection of ad blocks. I've already mentioned placement, and you may already have seen the "Heat map". You might find that in the one spot one of the available block types works better than another for you. It's a process of experimentation to find out what is most effective.