MER (marketing efficiency ratio)
MER is total business revenue divided by total marketing spend, taken straight from the P&L. Unlike ROAS, it cannot be double-counted or gamed by attribution.
Every platform claims conversions it touched, so per-channel ROAS figures typically sum to more revenue than exists. MER sidesteps the entire attribution argument by only comparing money that genuinely came in against money that genuinely went out — which makes it the referee when platform numbers and profit disagree.
The practical setup: derive a MER floor from blended margin and overhead (the level below which the business is unprofitable regardless of dashboards), then watch MER monthly by spend tier. MER falls naturally as prospecting share grows; falling below the floor is the alarm.
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