Number of tv viewers,

[logic]

When they say so many million people watched a certain programme, how do they know that, ?
How do they get the information,?

[squarebear]

Because they survey a large number of families and work it out from that. Some houses have a device on their TVs which make a note of what they are watching and for how long.

[vulcan]

squarebear, I've often wondered about this device. Does each individual member of the family log on and off ? I assume it's more sophisticated than just registering when the TV is switched on/off.
[apologies to logic]

[hc4361]

They don't use surveys at all, that would be extremely unreliable.
BARB gathers the data by the viewers that volunteer to have a monitor in their homes.
http://www.barb.co.uk/about/tvMeasurement?_s=4

[squarebear]

I don't know to be honest. I just assumed it was a recorder that was left on all the time and monitored what they were watching. If the TV is off, it records nothing.

[logic]

Hmmm,
Interesting, I always thought as SQUAREBEAR said, but i had never heard of anyone taking part in a survey,
But HC's answer seems very conclusive,
thanks guys.

[squarebear]

Sorry, by survery I mean an automatic electronic one taken by the device in question.