Every decision in ad management rests on a number: which creative won, which audience is efficient, where the budget should move.
If those numbers are wrong, all the decisions are wrong. And it’s hard to notice they’re wrong — because the dashboard always shows you a figure.
In most of the accounts we audit, at least one measurement error turns up. Below are the most common ones and how to check for them.
1. Double counting
The most common and most expensive mistake.
Sales data reaches Meta through two channels: the browser pixel and the server-side Conversions API. If deduplication by order ID (event_id) isn’t set up, Meta counts the same sale twice.
The result: the dashboard shows 4.2 ROAS when the truth is 2.1.
The check: Events Manager → data source → Purchase event → the “Event Deduplication” section. If the match rate is low, or it says “no deduplication”, you have a problem.
The rough check: For the same date range, compare the number of sales in your Meta dashboard with the number in your e-commerce dashboard. If the gap exceeds 20%, investigate.
2. The wrong event is firing
If the add-to-cart button fires the Purchase event, Meta treats every add-to-cart as a sale and distributes budget accordingly.
The check: Install the Meta Pixel Helper extension and walk through the site in order:
- Homepage → PageView only
- Product page → PageView + ViewContent
- Add to cart → AddToCart
- Checkout page → InitiateCheckout
- Thank-you page → Purchase
Any extra event firing means the setup is wrong.
3. No value is being sent
The Purchase event fires but carries no amount, so ROAS can’t be calculated. The dashboard shows the number of sales but no revenue — or always the same figure.
The check: Events Manager → Purchase → recent events → do the parameters contain value and currency?
A common mistake: not knowing whether the value is sent with or without VAT. Decide which it is and keep it consistent.
4. The thank-you page can be refreshed
If a user refreshes the thank-you page, Purchase fires again. On some sites a person refreshes three times and three sales get counted.
The check: Complete an order, refresh the thank-you page twice, and see how many Purchase events land in Events Manager.
5. Returns aren’t deducted
The ad platform doesn’t know about returns. If you have a 25% return rate in apparel, the revenue you see in the dashboard is a quarter higher than reality.
The fix: Don’t take the dashboard figure at face value. Take your real monthly revenue from your own back office and work out a correction factor.
6. The attribution window is confusing you
By default Meta uses a “7-day click, 1-day view” window. So someone who saw the ad, didn’t click, and 20 hours later arrived from Google and bought is still credited to Meta.
That isn’t an error, it’s a choice. But if you read it without knowing, you’ll think Meta is doing better than it is.
The check: Make the attribution window visible in your column settings. Put the “1-day click” and “7-day click” figures side by side. If the gap is large, Meta’s contribution is smaller than you thought.
The 20-minute checklist
Do this once a quarter:
- Walk the site page by page with Pixel Helper and verify the events
- Check the Event Deduplication rate
- Confirm
valueandcurrencyare on the Purchase event - Refresh the thank-you page and check for duplicate records
- Compare Meta’s sales count with your own back office
- Choose your attribution window deliberately
Last word
Measurement is the boring side of ad management. Nobody wants to deal with it; everyone wants to talk creative and campaigns.
But in an account that measures wrongly, even the best creative gets pointed in the wrong direction. First you fix the scales, then you weigh.