A source of truth is the one system a business agrees to treat as correct for a particular number. Revenue might come from your accounts package, leads from your CRM, ad spend from the platforms’ invoices and website behaviour from GA4. When two systems disagree, the source of truth settles which figure goes in the report.
How a source of truth works
The key idea is that it is chosen per metric, not per business. No single tool is right about everything. GA4 is good at what happened on the website but loses visitors who decline cookies. Meta and Google Ads count conversions using their own attribution rules and modelling, so their figures overlap. Your CRM knows which enquiries were genuine and which became customers, but not which advert they saw.
A workable setup usually looks like this:
| Question | Typical source of truth |
|---|---|
| How much revenue did we take? | Accounts or ecommerce platform (Xero, Sage, QuickBooks, Shopify) |
| How many real leads came in? | CRM or enquiry inbox |
| How much did we spend on ads? | Ad platform billing |
| Which channel brought the visit? | GA4 |
| Which campaigns should get more budget? | Platform data, checked against the CRM |
Everything else is then read as a direction rather than a final figure, and the gaps between systems are tracked through reconciliation rather than argued about.
The choice should also say what each number includes. A lead in the CRM might mean any form submission or only those a salesperson has qualified. Revenue might be invoiced or paid, gross or net of refunds. Writing the definition next to the system stops two people reading the same figure differently.
Why it matters
Without an agreed source, every meeting starts with a debate over which number is right. Picture a month, with made-up figures, where Meta says 60 leads, GA4 says 41 and the sales team counted 33. All three can be correct by their own rules, which I explain in why GA4 and Facebook conversion numbers do not match. The real risk is that each person quietly picks the number that suits their argument.
UK businesses hit a specific version of this with VAT. GA4 ecommerce revenue often includes VAT and sometimes delivery, while the figures in Xero or your management accounts are usually net. Unless you decide which one counts, return on ad spend can look a fifth better in one report than another for no reason except tax.
Common mistakes
- Naming one tool as the truth for everything. GA4 is not a finance system and an ad platform is not an independent judge of its own performance.
- Using platform-reported conversions as revenue. Add every platform’s claims together and you will usually exceed what you actually sold.
- Never writing the decision down. The agreement fades and the arguments return.
- Expecting systems to match exactly. A steady gap is normal. A gap that suddenly changes is the signal to investigate.
How to act on it
List the five or six numbers your business actually makes decisions on. For each one, name the system that holds it, who owns it and whether it is gross or net of VAT. Put that list at the top of your monthly report or measurement plan. Then record the usual gap between each pair of systems, so you notice when it moves.
Agreeing which numbers to trust is one of the first things I do in my digital marketing strategy and consulting service, because a budget plan built on disputed figures does not survive its first review.