ROAS or MER?

The ad platform reports one ROAS, the accounts tell a different story. When the two diverge, the argument usually starts with “which tool measures correctly”. Wrong question: both measure correctly, they measure different things.

What each one describes

In-platform ROAS divides the revenue a platform attributes to itself by the spend given to that platform. It supports campaign-level decisions: is this ad set better than that one? MER — marketing efficiency ratio — divides total revenue by total marketing spend and answers one question: is this business making money from marketing?

• ROAS depends on the platform’s window, attribution model and conversion definition; change the window and ROAS changes.
• MER depends on no attribution model at all; it is a division, not an attribution. That makes it blunter but much harder to game.
• ROAS is for comparing inside a channel; MER is for comparing across channels and over time.

Where they come apart

If the dashboards’ ROAS looks healthy while MER falls, there are three usual suspects: channels claiming each other’s sales, organic revenue being credited to ads, or returns and discounts pulling revenue down somewhere the dashboard cannot see. All three are real, and none of them is fixed by a dashboard setting.

When to read which

If the question is “which ad set should I switch off”, the answer lives on the ROAS side. If the question is “should I raise the ad budget”, it lives on the MER side. The difference is scale: the first question sits inside the campaign, the second inside the business.

In practice, keeping both numbers side by side works better than choosing one. ROAS drives day-to-day decisions; MER answers the weekly question of whether the overall direction is right. If they move in opposite directions for a long stretch, the problem is not reporting — it is either attribution or margin.