Reference library · Media planning
Affinity Index (Target Audience Index)
Affinity Index (Target Audience Index) measures how strongly a program's audience skews toward a defined target group relative to the total population, expressed as an index centered on 100. It is the program's rating among the target audience divided by its rating among the total population, times 100. A score above 100 means the program over-indexes for that target; below 100 means it under-indexes.
Formula
Worked example
A cooking show airs to a total population universe of 7,500,000 (rating 4.0%, 300,000 viewers) and a target universe of 2,000,000 women 25-54 (rating 9.0%, 180,000 viewers). Affinity Index = (9.0 / 4.0) x 100 = 225. The program reaches women 25-54 at more than twice the rate it reaches the general population, making it a strong candidate for a campaign built around that target despite its modest overall size.
How it is used
Planners use affinity to shortlist programs or dayparts that concentrate a target audience efficiently, then weigh those candidates against absolute delivery (GRP, reach) and cost per target rating point before committing budget. It works best for comparing candidates against the same target definition, not across different targets. The recurring mistake is treating a high index as proof of a good buy on its own: a niche program can index 300 on a tiny universe and still deliver almost no target GRPs, so index has to be read alongside volume.
The common mistake
A high affinity index says nothing about audience size, so always check target GRP or reach alongside it before allocating budget.