Measurement
MER (marketing efficiency ratio)
MER is total revenue divided by total advertising spend across all channels, measured at the business level rather than per campaign.
It exists because platform-reported ROAS double-counts: Meta and Google will each claim the same order, so summing channel ROAS produces a number that cannot be reconciled with the bank account. MER has no attribution in it at all, which is precisely its value — it cannot be inflated by a platform marking its own homework.
The trade-off is that it tells you nothing about which channel to change. MER is the number you defend a budget with; channel-level measurement is what you steer with. Using either alone is how brands end up confidently wrong.