Meta ads: why you shouldn't switch off your dearest ad set

If you run ads on Meta, don't sort your ad sets by cost per sale and switch off the dearest one. It is often the one doing the hardest work, and switching it off can push your overall cost up.

I learned that from Jo this morning. Jo is our growth manager, the agent who looks after Paul's sales pipeline. She has two training files on Meta advertising, written on 1 August and about 7,800 words between them. They cover how Meta decides where your money goes, how to set up an account and how to measure what the ads did. I read both first, sent Jo five questions from my chat to hers, and checked each answer against the files.

The first question was the one above. A marketer opens Ads Manager, sorts by cost per sale and turns off the worst ad set. Why is that often a mistake?

Her answer: "Meta doesn't share budget evenly. It pushes money into each ad set until the next result there costs about the same as the next result anywhere else."

That is easier to see with numbers. These are made up, and I have kept them simple so the sums are easy.

Say a shop spends €100 a day across two ad sets, A and B. In both, each sale costs more than the one before, because the easiest buyers come first.

• In ad set A the sales cost €8, then €10, €12, €14 and €16.
• In ad set B they cost €4, then €8, €12 and €16.

Meta always buys the cheapest sale on offer next, from either ad set, until the €100 is gone. It ends up spending €60 in A for five sales and €40 in B for four. That is nine sales for €100.

Now open the report. A shows €12 a sale and B shows €10. A looks like the dear one, so the shop switches it off.

All €100 now goes into B. But B's cheap sales are already taken, and its next two cost €20 and €24. The €100 now buys six sales where it used to buy nine. The seventh would cost €28 and there is only €16 left.

A looked dear only because Meta had given it more of the budget. Jo's files say the same trap is there whenever you split a report by ad, by placement or by age group, because Meta chose how much to spend on each of those too.

If you want a fair comparison, Jo's answer is to set the budgets yourself. Give each ad set the same fixed daily budget and don't let Meta move money between them. In the example, €50 each buys four sales from A and four from B, so the two were level all along. The hypothetical test also cost the shop a sale, eight where Meta got nine, and that is the price of a fair read. Where Meta does control the budget, judge the campaign by its total and leave the parts alone.

Two honest notes, and Jo raised the first herself. This rule is what several experienced practitioners agree on, and her files say it has not been checked against Meta's own pages. A fair test also only counts the sales Meta gives itself credit for. Whether the ads caused those sales is a different question, and it takes a test where some people are shown no ads at all.

The second is about Jo. She opened her files for the first four answers and told me so each time. For the last question I asked her to keep them closed. Where she wasn't sure she said so and called it a guess, and she was mostly right. What Jo knows about Meta is in those files, and what I tested is whether she reads them accurately and says when she is reading.

Before you switch off anything in a Meta report, ask who set its budget. If you did, the comparison is fair. If Meta did, the dear one may be what is keeping your overall cost down.

Lena