DefinitionA group overindexes when it appears more often in an audience than its share of the population would lead you to expect. Underindexing is the mirror image, and often the more useful of the two.
Always a comparison
The term is used for every variable an audience can be cut by: age, gender, region, interest, media channel, brand. It is always a statement about a comparison, never about a raw share.
Example (illustrative): a cycling audience that is 22% women overindexes on women if the baseline is 14%, and underindexes if the baseline is 31%. The 22% on its own says nothing.
Why underindexing is often more actionable
- It shows absence. A brand that underindexes in a region where its category is strong usually has a distribution or awareness problem, not an audience problem.
- It sizes the headroom. The gap between an underindexing group and the baseline is volume a campaign could realistically win.
- It protects against preaching to the converted. Budget tends to flow to overindexing groups because they look like success in a report, even where the additional return is low.
Common mistakes
- Points vs index. A move from 2% to 4% is +2 percentage points but an index of 2.0x: small in volume, large in lean.
- Cutting until something overindexes. Enough slices will always produce an extreme value somewhere. Decide the cuts before you look at the numbers.
- Ignoring base size. Publish the absolute number next to every index. A lean computed on a few hundred people is noise.
How Rascasse uses it
In Rascasse, every audience shows where it over- and underindexes: by region and city, by demographics, by interests, brands and media, each against the population of the same country. Regions are compared down to postal code where available.
Related use cases: Audience Profiling, Local Targeting & OOH, Media Planning.
Related terms
Affinity indexAn affinity index shows how much more, or less, common a behaviour is in one group than in a reference population.Regional affinityRegional affinity shows where inside a country an audience is concentrated, measured against each region’s share of the population.Audience sizeAudience size is the estimated number of people in a defined market who show sustained interest in a brand, club, artist, event or topic.