To quote from mSO, since my request is pretty much identical:
I would like to have statistics for the quality of posts coming from unregistered versus registered accounts on my site.
It is my suspicion that people who are willing to put out the effort to register an account will also put more effort into writing good questions; I'd like to see some evidence for or against this.
I also suspect that on my site the vast majority of good questions and answers come from registered users, but again I'd like to see some data.
Is this already possible through the Data Explorer? If so, how? If not consider this a feature request.
The reason for this is I am considering proposing that Server Fault stop accepting questions from unregistered users, and I would like to have statistics to analyze as part of that process.