Blended MER vs Platform ROAS: Which Should Drive Spend?
Platform ROAS is graded by the platform. Blended MER is the one number nobody can inflate for you. When to use each, and why MER is the honest lead metric.
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By Joris van Huët, Founder & CEOUpdated 3 min read
Run the numbers for your store: the free blended ROAS (MER) calculator.
Two metrics, two very different levels of trust. Platform ROAS is reported by the same platform selling you the ads. Blended MER, total revenue divided by total ad spend, is arithmetic on your own numbers that no vendor can inflate on your behalf. If you are deciding which metric should lead your budget, that difference in who grades the homework is the whole story.
Joe puts his finger on why platform ROAS lulls you to sleep:
"It's showing you a multiplier that it's working effectively, should be telling you that you're printing money, but you're watching this video because you have your doubts."
A number that is never negative and always says "print more" is a sales pitch, not a measurement. Blended MER is the one you compute yourself, so no platform can dress it up.
The definitions
Platform ROAS is channel-level revenue, as the channel claims it, over that channel's spend. Blended MER is all of your revenue over all of your ad spend, across everything, in one ratio. ROAS is granular and flattering. MER is coarse and honest.
Where platform ROAS breaks
ROAS breaks in exactly the places attribution debt lives. It double counts across platforms, it rewards demand harvesting, and it makes retargeting look like your best channel because retargeting catches people who already decided. Optimize hard to platform ROAS and you can improve the number while the business stops growing.
Why MER resists inflation
MER cannot be double counted, because it never assigns a sale to a channel in the first place. It just divides real revenue by real spend. No platform can pad it, because no platform is involved in computing it. That is why MER is the right lead metric: it is the number that moves only when your actual business moves.
The limit of MER, and what comes next
MER is honest, but it is blended, so it cannot tell you which channel caused the outcome. Two brands with identical MER can have opposite channel realities. MER tells you the machine is running well or badly; it cannot tell you which lever to pull. Answering that requires causal measurement at the channel level, which is where blended metrics hand off to incrementality.
Takeaway: Lead with blended MER because nobody can inflate it. Use platform ROAS with suspicion. Reach for causal measurement when you need to know which channel to move.
Watch the breakdown above, or on YouTube. See the €2,500 version of this problem in The €2,500 Question Every Store Owner Dreads, the evidence in What Happens When Companies Actually Test Their Ads, and the plan in The 3-Tier Framework.
Frequently asked questions
Should I use blended MER or platform ROAS to drive spend?
Lead with blended MER, total revenue over total ad spend, because no platform can inflate it. Treat platform ROAS with suspicion. Use causal measurement when you need to know which specific channel to move.What is the limitation of blended MER?
MER is honest but blended, so it cannot tell you which channel caused the outcome. Two brands with identical MER can have opposite channel realities. Channel-level answers require causal measurement.
Go deeper: Causal attribution, explained.
Sixty-second versions of these ideas: Causality Engine on YouTube Shorts.
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Terms in this article
- Ad SpendAd Spend is the total amount invested in advertising campaigns. It is measured against Return on Ad Spend (ROAS) to evaluate campaign effectiveness.
- AttributionAttribution identifies user actions that contribute to a desired outcome and assigns value to each. It reveals which marketing touchpoints drive conversions.
- Attribution DebtAttribution debt is the gap between what your ad platforms claim drove revenue and what actually caused it, carried quarter after quarter into the budget. It is how marketing debt accrues: allocate on claimed conversions long enough and the plan itself becomes the liability.
- IncrementalityIncrementality is the part of an outcome, such as orders or revenue, that a marketing activity caused: the extra results compared with what would have happened without it. It is a causal effect, not the number of conversions credited to a channel.
- MetricsMetrics are quantifiable measures that track and assess business process status. They evaluate campaign performance and inform attribution analysis.
- RetargetingRetargeting is online advertising that targets users who have previously interacted with your website or content. Attribution analysis shows the causal role of retargeting in driving conversions and improving ad spend.
- RevenueRevenue is the total income generated by the sale of goods or services related to a company's primary operations.
- ROASROAS: Return on Ad Spend measures the revenue generated for each dollar spent on advertising. Causal attribution calculates an accurate ROAS by showing the true impact of ad campaigns.