Affinity index
How much a medium's audience over- or under-represents your target, indexed to 100: an affinity of 130 means the channel's audience contains 30% more of your target than the population does. The media planner's tool for buying concentration instead of raw tonnage.
Where it's used
Used to choose TV channels, magazines, podcasts and sites. Good for finding concentration, read together with reach, because small channels often score highest.
How to calculate it
Affinity index = Target's share of the channel's audience ÷ Target's share of the population × 100
Worked example
Illustrative numbersWomen aged 25-34 make up 8% of the population and 14% of a home-design magazine's readers.
Affinity index
14 readers ÷ 8 population × 100 = 175
What it tells you
The magazine has 75% more of your target than the population does. If it reaches only 1% of them, though, it can add to a plan but never carry one.