ROAS relies on attributed advertising revenue. MER divides total business revenue by total ad spend.
Platform ROAS is useful for comparing campaigns inside a measurement system. It can overstate combined impact when several platforms claim the same customer journey.
MER avoids channel-level attribution but includes revenue influenced by brand, organic traffic, email, retail, seasonality and repeat customers. It is a business trend, not a causal measurement.
Use both: ROAS for campaign diagnosis, MER for blended efficiency, and experiments or geo tests when incrementality is the decision.
Sources
This guide is educational and does not provide financial, accounting, tax or legal advice.