// glossary
mROI (Marginal ROI).
Marginal Return on Investment
mROI (marginal ROI) is the return on the next unit of spend on a channel. Even with high average ROI, low mROI means the channel has hit saturation — adding budget no longer makes sense.
// detail
The most common budget-allocation mistake is deciding on average ROI. A channel with 4x average ROI sitting near the saturation elbow may produce sub-1x returns on additional spend.
Compare two channels:
- Channel A: average ROI 4.0x, mROI 1.2x → near saturation, don't add budget.
- Channel B: average ROI 2.5x, mROI 2.3x → far from saturation, add budget.
Channel A looks more profitable on average, but the next dollar earns more on Channel B.
mROI is read directly from MMM (Marketing Mix Modelling) outputs — the tangent on the saturation curve. Geo-lift testing provides experimental mROI: 'we added 30% spend on Channel X; sales rose Y%; mROI = Y/X.'
Example: A brand's average Google Search ROI is 5.0x. mROI analysis returns 1.4x — saturated. MMM shows Meta's average ROI is 3.0x but mROI is 2.8x. Shifting budget from Google to Meta (even though total ROAS appears to drop) produces incremental sales.