It seems to me that there are three basic formats for American situation comedies. Not every single sitcom ever made can be fit into these formats, although I am guessing most of them can. The first format is one in which an off the wall character is surrounded by a cast of relatively sane characters. The perfect example of this is I Love Lucy, which centred on the rather zany Lucy Ricardo (Ethel almost always went along with her schemes, although it was usually against her better judgement--one has to think she needed Lucy to keep her life interesting). The second format is in which every single character is off the wall. The perfect example of this is The Addams Family, on which every one of the major characters is not exactly normal. The third format is the exact opposite of the I Love Lucy format--one in which a relatively normal, rational character (the voice of reason) is surrounded by a cast of zany characters. The perfect example of this is The Beverly Hillbillies.
On The Beverly Hillbillies every character except one was not precisely sane. Granny was convinced the South had worn the War Between the States, brewed moonshine (well, she called it "rheumatism medicine"), and had a temper that was out of the world (not to mention she was at times delusional...). Jethro was a total dimwit, but was convinced he was a genius because he'd graduated from sixth grade. Elly Mae was a but overly attached to her animals and a total tomboy. Mr. Drysdale was wholly motivated by greed. And Miss Hathaway was an erudite, but man crazy spinster. Of all the characters, only Jed was sane and rational. Okay, he insisted on remaining true to his customs of the hills, down to still dressing as he did back into the hills, but he was not only the most intelligent character nad the only one who acted as a voice of reason. It is for that reason that I refer to the voice of reason on sitcoms with only one rational character as "the Jed Clampett character" or "the Jed Clampett figure."
Here I must point out that I have sometimes heard that "the Jed Clampett character" referred to as "the Mary Richards character" at times, after Mary Richards on The Mary Tyler Moore Show. I believe this is erroneous myself, as Mary Richards had more than her fair share of neuroses and insecurities. I think, then, there is no "Jed Clampett character" on The Mary Tyler Moore Show and the series in fact fits into The Addams Family format.
At any rate, there is no shortage of series which fit into The Beverly Hillbillies format. Indeed, one example was another show, like The Beverly Hillbillies, produced by Paul Henning (although not created by him--it was created by Jay Sommers)--Green Acres. Most of the residents of Hooterville were rather off the wall. Eb was glib and not particularly bright. Mr. Haney was a total criminal, always on the look out for an easy mark. County agent Hank Kimball was utterly scatterbrained. Even Lisa, Mrs. Douglas, who was not born in Hooterville, was off the wall; she insisted on naming every single animal on the farm and insists on dressing as if she was on Park Avenue rather than on a farm. Only Oliver Wendell Douglas is sane and rational--the voice of reason in an otherwise unreasonable world.
Another example of a show with a Beverly Hillbillies format is Night Court. Judge Harry Stone presided over a courtroom filled with off the wall characters. Prosecutor Dan Fielding was not only lecherous, but wholly amoral. Defence attorney Christine Sullivan was a bit too honest and a bit too naive, not to mention overly sweet. Bull was a bit dim witted and child like. Roz tended to be bitter and a bit brusque. The people who were tried before the court could be downright insane. Only Judge Stone provided a voice of reason on the show.
As a final example of a show with a Beverly Hillbillies format I offer up The Simpsons. With the exception of one, the Simpsons as a whole tend to be a bit zany. Homer takes the "stupid dad" archetype to new highs (or lows, as the case may be), and tends to be very much the clown. Marge is overly trusting, easily fooled, and a bit of a Pollyanna, although intelligent otherwise. Bart is a total delinquent. Not only are most of the Simpsons off the wall, but so is the average inhabitant of Springfield. Barney is a total alcoholic. Ned Flanders is overly religious. Mr. Burns is not only the epitome of greed, but perhaps the single most evil character on the show. Only Lisa Simpson acts as a voice of reason on the show. True, she is a bit precious and a bit of an activist, but she seems to be the only character who behaves somewhat reasonably for the most part and who acts not only as the voice of reason, but the show's conscience as well. As I see it, part of what makes The Simpsons extraordinary is that its "Jed Clampett character" is a child.
Since the Eighties it seems as if most shows do not fall into The Beverly Hillbillies format. In fact, most seem to fall into The Addams Family format, with not one rational character on the show. An exception may be 30 Rock, on which Liz Lemon would seem to be the "Jed Clampett figure." Of course, even Liz has her share of neuroses (she has never had a relationship which worked), so even that might not be an exception. Regardless, it is a format which has had a long history and one that can boast several shows which fit into it.