Blended vs Per-Channel
Blended metrics average across all channels; per-channel metrics isolate one source so you can see what is actually driving performance.
Blended numbers hide channel quality. If Meta is losing money and Google is printing profit, the blended average might look fine while the business is quietly burning budget in the wrong place.
Per-channel tracking is the drill-down. You need both: MER (blended) to see the overall health, and per-channel ROAS and contribution to see where to shift spend.
The trap is optimizing per-channel numbers in isolation. Google often claims credit for sales that Meta started. That is why per-channel ROAS should inform allocation decisions, not replace the blended view.
Related terms
Return on Ad Spend · Marketing Efficiency Ratio · Contribution Margin