ChatterBank4 mins ago
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One way they use is to have a cross-section of the viewing audience's tvs connected to a box which stores the information of the channel and times they were watching. Once a day this information is transmitted by phoneline to the reasearch company for analysis. As I have only come across a handful of these in a number of years of tv servicing, it seems a pretty small sample which suggests some degree of inaccuracy, and doesn't take into account 'passive' watching where the tv is just left on all evening.
Smorodina is right. We have one of these boxes and you key in when you are watching the TV or not. I think there are 5000 homes on the pannel (thats about 15-20000 people). The people who run it can be found here http://www.atruk.co.uk/ Hamish