OzTAM chooses 3135 homes around Australia to be a part of their 'Panel'. they are chosen to properly represent our population in just a small sample, so thousands of interviews are conducted every year to get the perfect mix for the panel.
each of these homes then have a Unitam box installed in their house. this box records: which house member is watching (they have to sign on and off with remotes), the time, duration and date of watching, whether each tv is turned on or off and the tv audio signal.
every night the data collected by each house is sent to OzTAM over the Internet. a content matching process begins where the audio recorded is matched to reference pieces from the actual broadcast. this is to see if a show was watched live, as live (paused at some point but watched before 2am) or time shifted (recorded and watched up to 7 days later).
all this data is collated, processed, analysed and verified and then integrated with the schedule provided by each network. it goes under rigorous quality control both digitally and manually, the output is a database of minute by minute audience viewing data.
so at somepoint the numbers are multiplied to be a full represention of our entire population. obviously it's not 100% accurate, ideally every home in aus would have one, but with 3135 homes recording data, then compared to our population, it's the largest sample size for tv ratings and thus the most accurate in the world