HOW DO THEY WORK?

Audience impressions are taken from Main Roads traffic counts and ABS data. For example, in Perth the average number of occupants in a car is 1.45 people. Given a road where 50,000 cars per day pass then an audience of approx 75,000 people per day is the expected figure for that particular site.

GRPs can be calculated by dividing the number of daily impressions into the total population of the city to reach figure. For example, a population of 1 million based on the above site would have a GRP (gross rating point) of 13.3, ie 13.3 % of the city would see the site.

However it is considered reasonable that the same 13.3 % would see the site each day for the month. Therefore a spread of sites is necessary to reach the entire city. Some people use a measles map of the city (red dots spread evenly over the city map) until the desired GRP is totalled up, eg 10 sites at a GRP of 10 will in theory give 100% coverage of the city's population. Not true in the real world though.

In reality many  other factors need to be considered: double viewing diluting the GRP; the same people seeing the same message twice as 2 sites are located on the same road; the demographic of the audience you want to reach, eg a BMW ad in Balga or a first homeowner’s scheme in Claremont.