The ratio has a currency

Return on ad spend looks like one number. In an Egyptian account it is two numbers in two currencies, and nobody says which.

The top of the fraction is revenue. Pounds, from a shop that prices in pounds, sells to people who earn in pounds, and pays its staff and its suppliers in pounds. The bottom is media, and most of the platforms bill in dollars.

So the ratio has an exchange rate inside it.

Move the rate and the number moves with nobody touching the account. Same creative, same audience, same offer, same conversion rate, and a target of four becomes a target of five, or the other way, depending on which direction the pound went since the target was written. A number agreed in January is describing a different job by July. Nobody has changed a setting, and everyone in the weekly meeting is either a hero or a problem.

I have watched teams spend a month optimising against that and calling it performance. The reporting is honest in the narrow sense that the arithmetic is correct. It is silent about the fact that a good part of the movement came from the central bank rather than from the work.

The fix is not clever. Report the ratio, then report the two numbers under it, each in the currency it arrived in, and state the rate used. A fall becomes readable: revenue held and media got dearer, or media held and revenue fell. Those are two different problems with two different answers, and the single number offers neither.

An efficiency target in a market with a moving currency needs a date beside it and a currency under it. Until somebody writes both down, the team is being graded on monetary policy.