How do tv ratings work?

How do they work out the tv network ratings in Australia?

• How many people watch certain channels and tv shows.

• Answer:

  Via a company called Nielsen. That's why you always hear about Nielsen Ratings. There are about 1000 people in Australia with a special box on top of their TVs that records data of what is watched and how often. This data is sent to Nielsen. The households that are picked to have these boxes are hand-picked by Nielsen. They match the boxes up to households whose occupants match a certain demographic, and creating a sort of miniature Australia. Nielsen then make assumptions based on the data and apply it to the whole of Australia's population. There is a lot of debate over whether the system is a true representation etc but it seems to work pretty well.