This has been bugging me for a long time. Remember that “Who Wants To Marry A Millionaire” fiasco? Apparently it was viewed by ump-teen million viewers that night. And the new series “Survivor” (yes I watch some Fox alright?), well I read that last weeks episode was watched by more people than any of the other episodes. They give rankings on series like “Friends” or “The Simpsons” (am I giving away way too much information about my personality by my examples of TV shows here???) and they say the Neilsen Ratings showed that one show was number one this week and one show was number one last week, watched by X number of viewers. My question is…How Do They Know That? It makes me nuts. Everytime I change the channel are numbers going up and down on some mysterious super-computer that moniters all TV’s everywhere? What if I leave my TV on and go away for a week. Do I count as a viewer? AHHHHHHH…I could go on and on. It driving me nuts. HELP!
Well, I’m not quite sure how they get the instantaneous results (or the next day results, anyway), but having just undergone the Nielsen Family thing in my household I can answer part of this question.
The whole thing starts with a phone call from the Nielsen people, who ask if you’re willing to be a Nielsen Family. You say yes or no. If you say yes they send you a booklet for each TV in your house, and a dollar.
In exchange for the dollar they ask that you record the TV viewing of everyone in the household, on any given TV, any time the TV is on for more than 5 minutes. (Of course, they’re also hopelessly stuck in the 50s, as their booklet references “Male head of household” and “Female head of household” as though you have to have one or both. Roomie and I were a little puzzled by this.)
You record this information for a week. At the end of the week you seal up the booklet and send it back to the Nielsen folks, postage free. One presumes that they then record the information in a computer somewhere and spit out the results.
Of course, the only TV my roommate and I watch is Buffy, Angel, Futurama and Simpsons, so our booklet was pretty much one long line of “TV off” and I suspect they’ll discount our results as being out of the norm, but hey, we did our part.
The diary system described by dogsbody above is used by Nielsen during the sweeps periods, to supplement their normal ratings data collection.
Most of the time, Nielsen collects it’s data through the use of a set-top device called a People Meter. People Meters are installed in around 4,800 homes across the country. It can tell the Nielsen folks what channel your set (or cable box )is tuned to at what time, or even if you are using your VCR or playing a video game.
The Nielsen computers in Dunedin, Fla. collect that data every single night at around 2:00 a.m. by calling the People Meters, which are attached to a phone line. Thus, they are able to produce the overnight figures. They calculate both the rating, which represents the percentage of the total TV universe of around 93 million television households (TVHH); and also the share, or the percentage of sets actually in use which are tuned to a particular program.
Not to be argumentative, but we just sent our diary back last week, and it’s hardly sweeps time. 
Actually, dogsbody, it is sweeps–February, May, July and November. Only the February and November sweeps are the high-octane overpromoted ones you usually hear about, though.
Make that May and November, not February.
Thank you both soooooo much for the info. It is much apreciated as, like I said, this question nags me every time I see the ratings posted someplace. Although rationally I knew there was a logical explanation, it was as mysterious as Stonehenge to me. 
So thanks once again.
Jawofech
What I don’t understand is whether they expect people to be honest. I would expect stuff like PBS to be overreported, and porn to be underreported. Do they take this into account?
Lying was quite an abuse in the old days but not so much now.
Diaries were used to supplement the boxes. If they didn’t jive someone was lying.
Later Nielson found people leaving the set on and no one watching. Now they have boxes that can detect whether someone is in the room or not.
As a former Neilsen family, I can shed some additional light on this.
It started with a phone call.
Could we hook you up?
How much?
$50 for the hook-up and $1.00 per month per TV.
What do I have to do?
Nothing.
Sounds good.
They took 14 hours to rewire my house. They cut up my carpet and glued it back down. They took my TV’s and VCR’s apart. They put boxes that could have passed for props in a sci-fi movie about a backpack nukes behind my TV’s. Everytime I watched a channel for more than 3 straight hours, they called to see if I had watched.
After a year, I moved and they came over and spent 8 hours removing their stuff. They trashed my house, left carpet torn ($200.00 repair they didn’t pay) broke my VCR (They did buy me a new one) and left my other TV’s and VCR’s looking like they had been violated.
FWIW - I used to leave the TV on when something I liked was on even if I wasn’t watching, just to get it a good rating. (That Nickelodeon ratings bump a few years ago - I did it.)
Lesson learned: there’s no such thing as a free lunch, and the Neilsens can be rigged if you really want to.
I wondered about that. So why do all of the really good shows always go off the air after a season or two? I’ve always figured that the demographics of the Nielson households are skewed towards people with no taste. (No offense intended to Dogsbody or Cynical)