They don't know and they have no way of knowing *exactly* how many. Ratings are done by statisticians based on limited samples from surveys that are extrapolated to include the rest of the population. There really is a lot of good research, math, and even science behind these statistical methods, so they should be reasonably close, but never EXACT.

Television viewership is figured in two basic ways. The first is by "ascertainment" -- that is, phone calls and surveys in which the audience members are asked what they watched. From this, it is assumed that if x percent of the sampled audience watched the show, then the same proportion of viewers in the larger audience watched the show. This is useful on a limited scale because it alos allows for follow up questions about whether the audience liked the show. Formal ratings are similar. A company (Nielsen, for example) takes a look at the map of the country and then the demographic properties of each area. It then selects people who represent their geographical and demographic group and asks them if they want to join and have their viewing habits analyzed. It used to be that members had to fill in diaries and submit them, but now, users can hook up boxes which monitor their use and are tied in to the company's central offices for instant analysis. The same assumption as above is made -- if I represent married, white, northeast, family head, and I watch a certain show, then all people in my demographic watch the show. The ratings are then represented by 2 numbers -- both percentages. One is called the "rating", which is the percentage of people (based on that extrapolation) who watched a show out of the number of people who own televisions , and the other is the "share" which is the percentage who watched the show out of the number who were watching ANYTHING at that time. The hope is that Nelsen has a large eogh sample that it can really reflect th overall population. Then Nielsen tells TV stations "we have this great data which will hep you assess the quality and popularity of your shows -- subscribe to our service and benefit!" The networks and stations may (and usually do) sign up and pay for the ratings books and use the data (or some elements of it) to tout their shows to advertisers.

now the set top boxes have come and this will give exactly how many viewers are viewing a particular channel at an instant time !! and that is the main reason the channels are raising against those set top boxes !!!

First, its all about money. Direct tv, comcast and your local cable company, run their entire operation through computers. this is done for several reasons. one, to control paying customers and non-paying customers. in other words, if you do not pay the bill, you lose your cable with the use of a computer and a flip of a button. two, computers keep tabs on what channel and what programs you are watching. this is money for the cable companies. Television stations pay money to the cable companies for this information. television stations base their commerical fees on this information. this is a win win situation for the cable companies. this is also how a television station can declare that they are number one in news, weather, etc. If you have cable and you thought what you watched was a secret... think again! what you watch is money in the bank to the cable companies and the television stations. Keep on watching! we are making them rich..er!

There is a rating service called "Nielsen" that all of the major television networks use. By using a combination of set-top boxes, diaries and other polling techniques, Nielsen can determine who's watching what on television and when they're watching it. They release a ratings report weekly that lists the top broadcast shows (though there are hourly and daily reports as well), and it's usually printed in the newspaper. Find out more at www.nielsenmedia.com.

Okay everyone. This is pretty interesting, so to summarize this topic I think is fair to state the following: "There is not way to know the "EXACT" ratings for a certain TV station. The customer how do not owned a Satellite service or cable service would have to be surveyed via phone or be participants who have boxes to survey their channels (not a satellite or cable box) and even when using this system they still do not know HOW MANY PEOPLE are watching the specific channel. Now on the satellite and cable customers they will know “EXACTLY” what you are watching but once again they do not know how many people are watching. I believe this is the reason why I never hear a station saying “Yesterday 27 million viewer tuned in” but they say things like “87% of the viewers prefer Fox” because they probably cont each “customer” as one. Well this is my 2 cents. Thank you Visit Iwonderwhy.net for cool topics!

They don't. They use random samples in statistical models to project as accurately as possible,