How do they figure our Australian TV ratings

From what I have learnt from the OzTAM website (the official supplier of TV Audience stats), this is how I believe it works. They use a sampling technique.

There are about 3000 peoplemeters out there (all states, all demographics) that record who is watching what (apparently they record tv watching every minute, every day). They then send that info back to the oztam head office who create an average audience for each channel for each minute. They then sum up the total for say “The apprentice” (sum those that watched Ch9 between 9:30 Pm to 10:30 PM for at least a minute divided by 60 minutes). So if 20 people watched for the full 60 minutes, the equation would be 20*60/60 = 20 people. If 10 watched for 20 minutes and 10 watched for 60 minutes, it would be ((10*20)+(10*60))/60 = 13.33 people. In that way they get the percentages right. They then extrapolate that (based on the demographic data) and apply it to all those in Australia who have a TV…It creates the figures that are then published.

Of course, because it is a sample, there is a sample variance (or error) which is conveniently left off the figures (dodgy).Apparently, they are going to start with Timeshift reporting soon, which tries to pick up people who have recorded shows via PVR’s to watch later…they are left off the figures at the moment. Will be a little more accurate.