Sgt_Kickaxe - 6:32 am on Nov 9, 2013 (gmt 0)
No sir, that's not 100% true for all units in all positions. For example I have a text only ad unit in a higher CTR location who's earnings are decimated if I turn on images because the CTR on images is very low in comparison. I do agree however for ads in low CTR areas of your site, if you're getting a very low CTR on a unit then having image/text enabled is probably best.
but Google will penalize in terms of RPM earnings you if you select to run just text ads only. At this time, to be achieve the highest earnings 99/100 times you have to be opted into all ads types.
In general image ad units don't get as a high a CTR as text only units so there is a CTR tipping point between the two.
As for the A/B testing tool I find it useful so long as you only use a particular unit in one location of one site. If you re-used the same unit in multiple locations or on multiple sites then you won't get actionable data with the tool.