This index tells us how the target you are analyzing is different from the average American population, so what is distinctive of your audience. Affinity can fall in the following ranges:
0<Affinity<1: your audience is less interested in the item than the average US pop
Affinity=1: your audience is as interested in the item than the average US pop
Affinity>1: your audience is more interested in the item than the average US pop
Affinity Formula= (Reach of an item in the target)/(Reach of an item in the average American pop)
In the example above, we can see that those Interested in Outdoor Voices (our target) are 151.91 times more interested in Glossier than the Average US American. This is a significant percentage, which is why we use Affinity to filter by what is distinctive of your target compared to the average US!