How do T.V. Companies find out how many people watched a program and how did they predict how many people were going to watch the Olympic opening ceremony?
They do this by using a panel of about 5000 homes which are chosen to be representative of each TV region. These homes are monitored for their TV use and the results are taken to represent the whole nation.
They are chosen from the results of a survey which interviews over 50,000 people a year. The survey is used to get general information about the UK population - family size, general viewing habits etc - and this is used to help select the most representative possible 5000 families for the panel.
If you agree to be on this panel all your viewing media are electronically monitored by a meter. You activate the meter by pressing a handset to indicate when you start watching a particular channel.
Olympic viewing predictions were calculated before the Games by using information from the 50,000 surveys.
You can learn more about the calculation and monitoring process here.