A campaign runs in March. Sales are up 8% versus February. Success?
Not necessarily. If the entire breakfast cereal category grew 10% in March due to seasonality, the brand actually underperformed the market. The 8% growth looked good in isolation but was 2 points behind the category trend.
This is why campaign results need to be indexed against the category, not just compared to the pre-campaign period.
What a category index does
A category index normalizes campaign performance against the overall category movement during the same period.
The formula: Brand Performance Index = (Brand Sales Change) ÷ (Category Sales
Change) × 100
If the brand grew 8% and the category grew 10%, the index is 80. The brand underperformed the category. If the brand grew 12% against a 10% category growth, the index is 120. The brand outperformed.
An index above 100 means the campaign beat the market. Below 100 means it didn't, regardless of how the absolute numbers look.
Why absolute growth misleads
Absolute growth is affected by everything: seasonality, weather, holidays, competitor activity, pricing changes, distribution shifts, macroeconomic trends. All of these affect the brand and the category simultaneously.
A campaign that launches in January, when health-conscious resolutions drive category growth, will show strong absolute performance simply by riding the seasonal wave. A campaign in August, when the category historically dips, will show weaker absolute performance even if it was more effective.
The category index strips out these external factors by comparing the brand's movement to the category's movement. If both moved the same amount, the campaign didn't change anything, it just rode the tide.
Category index and control groups
Category indexing and control groups serve complementary purposes.
The control group answers: "Was the campaign causal? Did exposed shoppers behave differently than unexposed shoppers?" That's an individual-level test of incrementality.
The category index answers: "Did the brand gain or lose ground relative to the market?
Did the campaign change the brand's competitive position?" That's a market-level assessment of strategic impact.
You can have a campaign with positive incrementality (control group test) but negative category index (the brand grew, but the category grew faster). That means the campaign worked at the individual level but didn't change the competitive landscape.
Conversely, you can have a positive category index with weak individual incrementality
, the brand's share grew, but it might have been driven by factors other than the campaign.
Both perspectives add to the complete picture.
The reporting standard
Every campaign report should include category-indexed results alongside absolute results and control group results.
Absolute: "Sales grew 8% during the campaign period."
Category-indexed: "Sales grew 8% vs. a category growth of 10% (index: 80)."
Incremental: "Control group analysis indicates 4% incremental uplift attributable to the campaign."
Three numbers. Three stories. Together, they give the brand a complete understanding of what the campaign achieved, relative to the calendar, relative to the market, and relative to what would have happened without the media.
The bottom line
Category index tells you whether you beat the category, not the calendar. Normalize performance against category movement. Otherwise, seasonality looks like success and headwinds look like failure.
It's one metric, easy to calculate, and essential for honest reporting. Without it, every Q4 campaign is a hero and every summer campaign is a disappointment, regardless of actual media impact.
Related Reading
- iROAS: The Retail Media Number That Makes Budget Decisions Easy
- Value Metric: What Retail Media Should Price Against
- Same-SKU ROAS vs Halo ROAS: Two Retail Media Stories, One Truth
- Incrementality: The Metric That Turns Retail Media Into a Growth Budget
- Rate Card Architecture: The Difference Between a Price and a System
Ready to see how this works in practice?
Footprints AI helps brands and retailers measure what matters. See our customer success stories or get in touch to discuss your retail media strategy.
