ROAS (Return on Ad Spend) has been the default performance metric for digital advertising for over a decade. It is simple, intuitive, and universally understood. It is also, in many cases, deeply misleading.
The problem with ROAS is attribution. Every platform reports the conversions it can claim credit for, using its own attribution model. In a multi-channel environment, this leads to significant double-counting, where the same purchase is attributed to Meta, Google, and email simultaneously.
The Attribution Problem
Consider a customer who sees a Meta ad, clicks a Google retargeting ad, and converts via an email. Meta reports a conversion. Google reports a conversion. Your email platform reports a conversion. Your actual revenue is one purchase, but your reported ROAS across channels suggests three.
This is not a hypothetical. For most multi-channel e-commerce brands, the sum of channel-reported conversions exceeds actual revenue by 30-80%. Decisions made on the basis of individual channel ROAS in this environment are systematically distorted.
What Is Marketing Efficiency Ratio?
Marketing Efficiency Ratio (MER) is calculated as total revenue divided by total marketing spend. It is a blended metric that measures the overall efficiency of your marketing investment without the attribution distortions of channel-level reporting.
Using MER as Your North Star
MER works best as a north-star metric that tells you whether your overall marketing investment is efficient. If your MER is 4x (meaning you generate $4 in revenue for every $1 in marketing spend) and it is stable or improving, your marketing programme is healthy.
Use channel-level ROAS for tactical optimisation decisions within channels, but use MER to evaluate whether to increase or decrease total marketing investment.
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