Question: I have a question that has been killing me for a long time. Every time the Nielsen ratings come up, they always display ratings for a certain show by age range. For instance, on a recent TV show, they said that it received the most viewers between the ages of 18-24. How on earth do they know how old the viewers are? How do they know if adults or teens watch their shows? While we are on the subject, how do they calculate the viewer numbers? Is our television some kinda tracking device? Basically, how do they know who is watching? — Johanna D.

Televisionary: Your TV isn't a tracking device, Johanna, but the boxes the folks at Nielsen place in sample families' homes certainly are. I've covered this before, but it's been a while, so what the heck? Others are probably wondering about this kind of thing, too. Here are the basics.

Nielsen uses a representative sample of U.S. TV households to determine who's watching what and when since covering all of the 99 million households with TVs is out of the question for obvious reasons. The sample is made up of thousands of houses and viewers randomly picked so that the mix represents a wide range of demographics and locales. Extrapolating from that, in theory, provides a picture of what the whole country is watching at any given time, within a certain range of error.

Exactly how do they measure? Using a few methods. They know the age, gender, etc. of each family member when they choose which households to work with. Meters are installed on every video device in the home — TV sets, cable boxes, VCRs, satellite dishes etc. — to record which shows each is tuned to. The company also uses "people meters" to keep track of which person in the house is watching and diaries and other methods are used to gather numbers on a market-by-market basis.

So yes, they're tracking how many members of each demo watch which show. And since TV is ad driven, ad buyers believe the 18-34 demographic is easiest to target and want to spend their money on shows which attack that age group, it matters. (As I've said in this space before, I think that theory's stupid since older people have more money to spend — recent studies and people who know a lot more about it than I do back me up on that — and I think we end up with a lot of crappy, pretty-young-thing TV because of a system of bad assumptions. But I don't want to rant.)