Advertising and marketing metrics
MER (marketing efficiency ratio)
Total revenue divided by total marketing spend across all channels. Also called blended ROAS. Avoids attribution arguments but hides channel differences.
MER is total revenue divided by total marketing spend across every channel for a period. It is often called blended ROAS. Because it uses the store's own revenue rather than each platform's attributed revenue, it sidesteps the question of which channel deserves credit for a sale.
That is its strength and its limit. MER is hard to argue with and easy to compute from two numbers an assistant can fetch: store revenue and summed ad spend. But it hides channel differences, so a rising MER can conceal one channel improving while another wastes money. It also uses revenue, so the same margin caveats as ROAS apply.
A common approach is to track MER as the top-line check, POAS or contribution margin as the profit view, and platform ROAS only for decisions inside one platform. With an assistant connected to the store, the ad accounts and analytics, all three can be produced in one conversation, which makes the differences between them visible rather than hidden in separate dashboards.