The Incrementality Question Nobody Asks (Until It's Too Late)
Your ROAS report shows £500k in attributed revenue. But how much of that would have happened without the ads?
The Question
Would this customer have purchased if we hadn't shown them this ad?
This is the incrementality question, and it's the most important question in marketing measurement. It's also the one that rarely gets asked until budget pressure forces it.
The Core Question
Attribution tells you who saw the ad before converting. Incrementality tells you whether the ad changed their behaviour. These are fundamentally different questions with different answers.
Why It Matters
Consider branded search. A customer who already knows your brand types your name into Google, sees your ad, clicks it, and buys. Attribution credits this entire sale to the ad.
But would they have bought anyway? They already knew your brand. They were actively searching for you. Without the ad, they would have clicked the organic result instead.
The attributed conversion is real. The incremental impact might be zero.
The Budget Implication
If 40% of your attributed conversions would have happened anyway, your true ROAS is 40% lower than reported. That changes which campaigns are profitable and how much you should spend.
Where Incrementality Hides
Some campaign types have lower incrementality by design:
- Branded search: Often captures demand that already exists
- Retargeting: Targets people who already know you and may be close to purchase
- Cart abandonment: Targets people who were already considering purchase
Higher incrementality typically comes from:
- Non-brand search: Introduces your products to new audiences
- Shopping prospecting: Shows products to people who weren't searching for you
- Discovery/Display: Creates awareness before active search begins
How to Ask It
The incrementality question should be asked at three levels:
- Channel level: What would happen to our business if we stopped advertising on Google entirely?
- Campaign level: Which campaigns are creating new demand versus capturing existing demand?
- Marginal level: What's the incrementality of the next pound spent versus the average pound?
Most advertisers never progress past channel-level questions, leaving significant optimisation opportunities on the table.
How to Answer It
There are several methods to measure incrementality, each with trade-offs:
- Geographic holdouts: The gold standard. Pause advertising in selected regions and compare sales to control regions.
- Conversion lift studies: Google's built-in tool that measures lift against a control group (requires sufficient volume).
- Ghost ads methodology: Compare behaviour of users who were shown ads versus those who would have been shown ads but weren't.
- Budget variation testing: Vary spend levels and measure marginal impact on sales.
Start With Branded Search
Your first incrementality test should be branded search because it's the campaign most likely to have low incrementality. If brand search shows 70% incrementality, your other campaigns are probably fine. If it shows 20%, you've found significant savings.
Next Steps
Don't wait for budget cuts to ask the incrementality question. By then, you've already spent money on conversions that would have happened anyway.