Why it is useful
The index removes size from the comparison. A segment of 40,000 people and a segment of four million can be read on the same scale, because both are measured against the same baseline. That is what makes it the standard way to say where an audience leans.
The formula
Two notations, one number
- Multiplier notation. 1.0x is the baseline, 2.0x is twice as likely. Common in digital and social analysis.
- Index-100 notation. 100 is the baseline, 200 is twice as likely. Standard in classical market research and media planning.
Both describe the same value, scaled by 100. An index of 120 and an affinity of 1.2x are the same finding. Mixing the two in one deck is a frequent reporting error.
The baseline decides the answer
An index is only as meaningful as the population in the denominator. The same segment can show 3.0x against the national population and 1.1x against people who already buy the category. Both are correct. They answer different questions.
- National baseline: is this audience unusual compared with everyone? Useful for positioning and for finding groups you do not reach yet.
- Category baseline: is this audience unusual compared with our existing buyers? Useful for deciding where additional budget goes.
An index without a named baseline cannot be checked. Send it back.
Common mistakes
- Small segments. With a few hundred people in a segment, a handful of individuals can move the index from 1.0x to 4.0x. Always read the index next to the absolute size.
- Circular definitions. The affinity of football fans for football boots is high and tells you nothing.
- Index without reach. A 6.0x index on 12,000 people is often worth less than 1.4x on four million. The index shows concentration, not scale.
How Rascasse uses it
Rascasse calculates affinities from AI-based analysis of aggregated digital behaviour signals, not surveys, so the baseline is an observed population rather than a weighted panel. Every audience is measured per country against that country’s population: a club has different indices in Germany and in the United States, and one is never used as a stand-in for the other. Each audience is described by 100,000+ data points. See how we measure.
Related use cases: Audience Profiling, Sponsorship Matching & Pitch, Segmentation & Personas.