Hi, I would apreciate a good advise regarding this: I sometimes need to lower bids in order to improve cost-effectiveness. When I do so, some af the Ad Group's keywords go "inactive for search". (Let's assume I've reached the highest CTR possible.) Those keywords should remain inactive (increasing the bid will bring losses), but on the other hand I'm afraid that their presence is lowering the total Ad Group's quality score. So should I delete those keywords? I tried to statistically examine this question but it's more complicated than it seemed in the first place
There's no ad group quality score, the quality score in this case is at the keyword level. So to answer your question - inactive keywords don't have a negative impact on other keywords in the account.
You should consider deleting keywords if they become inactive for search, as these keywords are performing poorly for your account. Keywords that are still active, but produce a relatively low CTR and high minimum bid, often can be improved. You can simply delete the keywords and add more specific, better targeted keywords instead.