ROAS
Return on ad spend
The revenue attributed to advertising divided by the amount spent on it, over the same period.
ROAS = attributed revenue / ad spend
What makes it wrong.
The mechanism, not the warning.
A ROAS pulled per platform and then added up double-counts. The overlap is not an error in either platform: each is answering the question it was asked, which is what happened inside its own attribution window. Only a shared source of truth on the revenue side, or an incrementality test, settles it.
In detail.
ROAS is the ratio everything else is judged against, and it is a ratio of two numbers that come from different systems. The spend is what the platform charged, which is close to exact. The revenue is what the platform decided to credit itself with, which depends entirely on its attribution rules.
Two platforms reporting on the same week will both claim revenue for the same order if both touched the customer, so the sum of platform ROAS across a business is generally higher than the business's real return.
Through an agent.
What changes when a model asks instead of a person.
Ask a model for ROAS across four platforms and it will happily sum them unless the tool descriptions say the values are not additive. This is the single most common wrong answer in cross-platform reporting.
Where this is measured.
ROAS comes from Meta Ads. The server page lists every tool that returns it, with its parameters.