Incrementality testing in practice: geo-lift and holdout
Nicklas Segatz Mortensen · Growth Hacker · Fractional CMO · Meta Ads Nerd · 8 July 2026 · 6 min.
01Why you can't trust attribution alone
Attribution distributes credit for sales that have already happened — it can't know whether the sale would have come without the ad. Retargeting and brand searches look fantastic precisely because they hit people who were on their way anyway. Incrementality asks the only question that matters: what would have happened without the effort?
The answer isn't found in Ads Manager, but in an experiment: hold something back, and measure the difference. That's the principle behind both geo-lift and holdout tests.
Sådan virker det
Inkrementalitet er forskellen mellem en gruppe, der ser annoncerne, og en holdout-gruppe, der ikke gør. Kun mereffekten — det grønne — er reelt skabt af annoncen. Resten var kommet alligevel.
02How to run a geo-lift test
Split your markets or regions into two comparable groups: a test group where the effort runs, and a control group where it's turned off or held flat. Run it long enough for the difference to be statistically credible — typically a couple of weeks to a month depending on volume. The difference in total sales between the groups is the incremental effect.
The advantage is that you don't have to trust cookies or windows — you're comparing actual revenue in two groups. Meta's Conversion Lift and Google's geo experiments automate parts of it, but the principle is the same: hold back, and measure the difference.
03What you typically find
Most brands running their first holdout test discover that some of their "best" attributed channels are far less incremental than the dashboard claimed — while the demand-generating top of funnel is more valuable than last-click gave it credit for. That changes how budget should be allocated.
From above, MER can give a cheap, ongoing check: if you raise budget and blended efficiency holds, the growth is incremental. Formal lift tests and MER complement each other — one precise and occasional, the other continuous and cheap.
Frequently asked questions
What's the difference between a geo-lift and a holdout test?+
Both hold something back and measure the difference. Geo-lift uses geographic groups (regions/markets), while a holdout typically keeps a random share of users outside the effort. The principle — measure the added effect against a control group — is the same.
How long should an incrementality test run?+
Long enough for the difference to be statistically credible — typically a couple of weeks to a month, depending on your volume. Too little data gives an uncertain result; too long costs unnecessarily in lost effort in the control group.
Related terms
Glossary
What is incrementality?
Incrementality is the added effect a marketing effort creates: the sales that happened only because the ad ran. Sales you'd have gotten anyway aren't incremental — whatever the platform credits.
Read the entry →Glossary
What is attribution?
Attribution is the method that distributes the credit for a conversion across the touchpoints the customer met along the way. The model decides which channel gets the credit — and therefore where budget flows.
Read the entry →Glossary
What is MER?
MER (Marketing Efficiency Ratio) is your total revenue divided by your total marketing spend across every channel. It ignores the platforms' own attribution and shows how efficiently the whole marketing machine is working.
Read the entry →Glossary
What is an attribution window?
An attribution window is the time period after a click or view within which a subsequent conversion is credited to the ad. Meta's default is 7-day click and 1-day view.
Read the entry →
Nicklas Segatz Mortensen
Growth Hacker · Fractional CMO · Meta Ads Nerd at Oaksmond
Growth hacker and fractional CMO with 10+ years' experience and hundreds of millions in managed ad spend behind him. Background from larger Danish and international scale-ups, and from the agency world.
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