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They probably need to be a tad more verbose than the OP…
I assume it’s known among the staff who has expertise in what areas, and assignments are given accordingly.
And, I’m given to understand, occasionally a member who is known to the staff and known to be expert in a field will be coopted to produce a staff report answering a question relating to his field of expertise.
Generally, Staff Reports are written by, ah, Staff. There are a few “guest” Reports. And actually, Revtim, there’s no real “assignments”, it’s more of a volunteer basis. Staff look over the questions and choose which ones they want to tackle. And “expertise” isn’t often an issue – we get questions where “expertise” doesn’t come much into play.
OK, sure, Karen tends to do physics and Doug tends to do bugs, but Euty tackled a question about porn stars. And next week, I’ve got one on fire investigations, about which I knew nothing before I tackled the question.
Yes, Polyc, occasionally we find a question and we know of a member who has expressed some expertise in that area, and we approach them to do a guest report. Truth be told, most of those haven’t worked out – initial agreement, then a long period of silence.
Bottom line: if you are interested in writing a Staff Report, email me your credentials and fields of interests, along with some indication of why you think you are suited (or why we should consider you.) If there seems to be a future in it, we might give you some to try out.
So, like they said, expertise 
All of the mods are automatically Staff, as well, which is how I ended up on the team. Of course, some of them write even less than I do… glares meaningfully in a few select directions… You know who you are
(I mean, hey, if some of those slackers started writing more, Dex would be on our cases less, right?)
Hey, my official job title is Czar-Autocrat and Bureaucrat Extraordinary of the Staff Reports… and the job is to nag or spur or needle y’all into writing Reports. That’s what Ed doesn’t pay me for, and that’s what I do.
And I’ll reveal an inner secret: a person with deep expertise in only one subject probably would not be invited as a Staff Member. They might write some “guest reports,” but we don’t get enough questions on Etruscan pottery (say) to put on Staff a person with enormous expertise in only that one field.
P.S. - If you do email me inquiring, please be sure that the subject line is clear. I get tons of spam email, and I tend to delete stuff without opening if I don’t know where it’s from or if the subject line is at all unclear.
Dex, you don’t know how tempting it is to forward you a couple of my latest “enlarge your penis” or “consolidate debt now” spams, being sure to change the subject line to “Dex I’d like to write about this for the Dope”!
Of course, that would probably place me on your fecal roster, so maybe I’ll just enjoy the thought - Heh.
There is a Fecal roster? hmm… I guess lieu is a consultant… 
ha 