Three metrics, three questions
ROAS divides attributed revenue by ad spend. MER, in the convention used here, divides total business revenue by total ad spend. POAS divides pre-ad contribution profit by ad spend. Definitions vary between teams, so agree on the numerator, denominator and treatment of returns before comparing reports.
ROAS: compare within its attribution limits
ROAS helps compare campaigns under consistent attribution rules. It does not directly show profit. Google and Meta may both claim a sale, and gross order values may include orders that are later returned. Adding the revenue from two ad dashboards can overstate store revenue.
MER: a broader view
If monthly store revenue is 100,000 and total ad spend is 20,000 in the same currency, MER is 5. This illustrative calculation avoids allocating each sale to a platform. It still does not prove that advertising caused all the revenue, and it does not identify which campaign needs a change.
POAS: define the profit carefully
Suppose 10,000 in ad spend produces 30,000 in revenue, but only 9,000 remains after product costs and other variable costs, before advertising. ROAS is 3 and POAS is 0.9. Advertising then leaves a negative 1,000 contribution before fixed costs. This is an illustrative example, not a customer result.
Use the metric that fits the decision
Use consistent ROAS comparisons for campaign diagnosis, MER for the broader efficiency trend and contribution economics for profitability decisions. Review returns, cost accuracy and customer mix. No single ratio proves incremental revenue or determines whether scaling is safe.