Google have always been keen to stress that organic search and paid search results operate completely independent of each other, and generally this is for the right reasons.
They’ve also constantly denied that user signals, in particular CTR, are a ranking signal, despite numerous tests suggesting otherwise.
I don’t envy the engineers who are responsible for evaluating user signals as a quality measure. There are so many factors to taken into consideration and so much misinformation out there. To give an example, a lot in the industry are fixated on the time-on-site metric, and bounce rate as indicative measures for quality of content. The thinking is that a long session length is a quality measure. This isn’t necessarily true. A short session length and high bounce rate could suggest quick information retrieval, and therefore be a quality signal.
CTR too, is difficult, and assuming this is a ranking signal, there needs to be some connection between organic and paid listings. Through changes over the years, Google have cleverly managed it so that paid search often can be demonstrated to have an incremental effect on performance. Lots of brands will run paid search on generics, and this is likely to reduce the CTR of the organic listing. That reduction in CTR isn’t a reflection on the quality website.
If brands are running paid search, unless organic and paid search algorithms are connected, Google has no robust method to evaluate CTR as a quality signal. The algorithms need to be connected, and the long standing position of their disconnected status needs to be re-evaluated.