Research Postar model What is VAC?

A visibility adjusted contact or VAC is the audience rating for an advertising panel.  It is not a measure of quality but simply one of the scale of the audience.  As such it is comparable to a television or radio spot rating.

How do we calculate a visibility adjusted contact or VAC?

The visibility model allows us to go beyond simply counting the number of people who pass a site and estimate those that will actually look at it.  Various factors such as the distance from the side of the road, illumination, the size of the structure and the angle of inclination to passing traffic will affect how noticeable a site might be.  This is a calculation of "eyes-on" an advertising panel.  It is not a measure of detailed reading or cognisance.  Such measures of “impact” are not within the scope of audience measurement.  

The research experiment is based on eye fixations.  Each subject wears a headset through which infra-red beams measure and record where the eye moves, where it fixates (focuses) and for how long.  The subjects see a wide range of scenes, both with and without poster advertising. The fixations are overlaid by computer onto the target posters in the photographs and a “hit” or a “miss” is recorded.  Data on the distance of the poster from the subject, its relationship to the kerb and the size and angle of the hoarding are all derived from the photographs using simple geometry. The resulting data is used to model a subject’s probability of fixating on a poster site when its location, angle to the road, size and so forth are known. Using the probability model in combination with the known traffic estimates, each of the 127,500 roadside posters can be assessed for their likely audience by adjusting the gross number of passages past by the visibility index for a panel of its type and characteristics. 

These estimates are further adjusted to account for daily traffic flows and the variable hours of daylight, providing a seasonally sensitive audience estimate that is adjusted for the likelihood to see.

The most common misconception of the research data is that VAC is a measure of a site's "quality".  It is not.  One panel having a higher VAC than another only signifies that it has a higher audience.  For many reasons, the panel with the lower audience may well have a greater impact on those that see it.