Treating PMax as an incremental channel means measuring the conversions it genuinely added — not the ones it claimed but would have happened anyway through brand, organic, or other channels. Because PMax harvests cheap conversions across all of Google, its reported numbers overstate its true contribution; incrementality testing (holdouts, geo experiments) reveals what it actually drove.
At a glance
- PMax reports conversions it claimed, not ones it uniquely added.
- Much of that is brand and existing demand it would have captured anyway.
- Reported ROAS overstates PMax’s true contribution.
- Incrementality testing reveals what it genuinely added.
- Judge PMax on incremental conversions, not claimed ones.
Performance Max almost always looks like your best channel, and that should make you suspicious, not happy. PMax reaches across all of Google and is exceptionally good at finding cheap conversions — including the ones that were going to happen regardless, through brand searches, organic visits, and demand other channels created. It claims them, reports a gorgeous ROAS, and earns more budget on the strength of conversions it didn’t actually cause.
The only question worth answering is incrementality: of everything PMax reported, how much would not have happened without it? That number is usually far smaller than the dashboard — and it’s the only one you should be budgeting against.
Claimed vs. incremental conversions
A conversion PMax reports and a conversion PMax caused are different things. The gap between them is where budget gets wasted on a channel taking credit for work it didn’t do.
| Claimed (reported) | Incremental (true) | |
|---|---|---|
| Includes brand harvest | Yes | No |
| Includes existing demand | Yes | No |
| Reflects true contribution | No | Yes |
| Right basis for budget | No | Yes |
Why PMax over-claims
PMax’s reach is the root of the problem. Given access to Search, Shopping, brand queries, and remarketing, it naturally scoops up the cheapest, highest-converting traffic — which is exactly the traffic that converts with or without an ad. Unchecked, it inflates its own numbers by harvesting demand other parts of your marketing already created, then presents that as performance.
Relative share of non-incremental claims.
How to measure incrementality
You measure what PMax added by withholding it and watching what changes. A geo holdout (PMax on in some regions, off in comparable others) or a time-based holdout shows how many conversions actually disappear without it — that’s the incremental number. Excluding brand from PMax first removes the most obvious inflation. The goal is a defensible read on contribution, not the platform’s self-report.
So should I stop trusting PMax’s reports?
Every channel wants credit for the conversion, and PMax is structurally positioned to grab more than its share. Measuring incrementality is how you cut through the self-congratulation and fund PMax for what it actually adds — which is the only honest way to size any channel.
Is PMax adding conversions — or claiming existing ones?
a month — about $105,600/yr — going to clicks that never convert.
Frequently asked questions
What is incrementality?
It’s the measure of conversions a channel genuinely caused — ones that would not have happened without it — as opposed to conversions it merely reported or claimed. For PMax, it strips out brand and existing demand it would have captured anyway.
How do I test PMax incrementality?
Run a holdout: keep PMax on in some geographies or periods and off in comparable ones, then measure how many conversions actually disappear without it. That difference is the incremental contribution, which is far more honest than reported ROAS.
Why does PMax over-report?
Its reach across Search, Shopping, brand, and remarketing lets it harvest the cheapest, highest-converting traffic — much of which would convert anyway. It claims those conversions, inflating its reported numbers above its true contribution.
Should I exclude brand from PMax?
Usually yes — brand harvesting is the biggest source of over-claiming. Separating brand out (or excluding it) removes the most obvious inflation and gives you a clearer read on PMax’s incremental, non-brand contribution.
Article by
Richard Castello
Richard leads performance and search strategy at PPC Snobs. He’s spent over a decade architecting paid acquisition engines for DTC and B2B brands — managing live budgets at scale, not recycled SEO filler or AI-only takes.