i have always made keyword decisions based on positive ROI... in other words, if a keyword was working for me, i would keep it in an adgroup regardless of the CTR.
it has occurred to me, however, that a keyword with a very low CTR is not working too well for google - in other words, they don't make too much money on it. does google penalize the other keywords in an adgroup if some of the keywords have a very low CTR?
if so, it seems that, contrary to common sense, it would be best to delete keywords (or move them to other adgroups) that have very low CTRs even if they have positive ROI.
just to make this clear, suppose that you have an adgroup with two keywords. both are giving you positive ROI, but one of the keywords has a CTR of 5% and the other has .2%. for the sake of this example, lets suppose that the low CTR keyword has a huge number of impressions thereby bringing the adgroup CTR down to say, .3%. so, the question is, would it ever make sense to delete the .2% keyword to bolster the 5% keyword?