Published 3 September 2026 · 5 min read
Before any spend analysis, someone has to decide which dataset to pull: purchase orders or invoices. They give different answers, and the gap between them is often the most interesting thing in the exercise.
The short answer
Use invoice or accounts-payable data as your primary source. It is what you actually spent. PO data is what you intended to spend, and the difference between intent and outcome is exactly the spend procurement has least control over.
What each one gives you
- Invoice / AP data. Complete — everything paid appears, including the off-contract buying that never had a PO. Reconciles to the general ledger, so finance will accept the totals. Usually thinner on description, which makes classification harder.
- PO data. Richer descriptions, line-level detail, often a category code already attached. But it only covers what went through the purchasing process — and the spend you most want to find is precisely the spend that did not.
The gap between them is the finding
Sum both for the same period. Invoice spend with no matching PO is your maverick spend, and in most organisations it is 15–40% of the total.
That single comparison is often more actionable than the category breakdown, and it takes an afternoon. It also tells you how much to trust a PO-only analysis: if PO coverage is 60%, a PO-based report is describing three-fifths of your spend and quietly ignoring the rest.
Combining them
The best of both is invoice data as the spine, enriched with PO descriptions where a match exists. Match on PO number if your AP records carry one, otherwise on supplier plus date plus amount with a tolerance.
Do not attempt this on the first pass. Get a classified invoice dataset working, then decide whether the enrichment is worth the matching effort — often it is not, because modern classification copes with terse descriptions better than the matching exercise costs.
Practical pitfalls
- Credit notes. Negative lines. Keep them, or your totals overstate.
- Header vs line level. Header-level invoices collapse a mixed basket into one supplier and one amount. Request line level if it exists.
- Intercompany. Usually excluded, but decide explicitly and write it down.
- Tax. Analyse net. Gross figures will not reconcile to the spend finance reports.
Further reading
- Preparing an ERP export — which columns to request.
- 7 spend data quality problems — what to check once you have the file.
- Maverick spend — what the PO gap tells you.