How many invoices and bills will a GHG verifier sample?
There is no fixed number. Under ISO 14064-3 the verifier sets a sampling plan from the level of assurance, the materiality threshold and the risk of error in each emission source. Large sources and high-value documents are often checked in full, the rest by sample, and reasonable assurance needs larger samples than limited assurance.
What sets the sample size
- Level of assurance: reasonable assurance needs more evidence, and so larger samples, than limited assurance. ISAE 3410, used by accounting firms, works the same way.
- Materiality: the size of error that would change a reader's view, often set at 5 percent of the total by the programme or the verifier. A lower threshold means more testing.
- Risk: sources with manual data, estimates, many small documents or past errors get more testing.
- Controls: if your process has checks the verifier can test and rely on, such as approvals and reconciliations, it may sample fewer documents.
- Population: how many documents there are and how much each one contributes.
How verifiers usually split the population
They often test every item above a set size, such as the few fuel invoices or bills that make up most of a source, and then sample the many smaller ones at random or by a structured method. A source with 12 monthly electricity bills may be checked in full, while a source with thousands of small fuel slips is sampled. The verifier records the plan and the reasons for it.
What makes a sample go badly
- A sampled document cannot be found, or the figure in it does not match the inventory.
- Errors found in the sample are projected to the whole population, so a few small errors can add up to a material one.
- Documents are photos with no link to the line they support, so each one takes time to trace.
- When errors appear, the verifier widens the sample, which adds cost and time.
How to prepare
- Keep every source document, indexed to the inventory line it supports.
- Reconcile each source to an independent record, such as fuel to the purchase ledger and electricity to paid bills.
- Fix known errors before the verification starts and note the corrections.
- Ask the verifier for its sampling approach at planning, so you know which periods and sources to have ready.
Worked example: how a fuel source might be split
- Diesel for the year: 500,000 litres x 2.68 kg CO2 per litre = 1,340 t CO2
- Materiality at 5 percent of a 1,340 t source: 67 t CO2
- 40 invoices over 5,000 litres each total 300,000 litres, 804 t CO2, 60 percent: a verifier may test all of them
- The other 200,000 litres, 536 t CO2, spread over many small invoices: tested by sample under the verifier's plan
Illustrative only. The verifier decides the split, the threshold and the sample, and none of these numbers is a rule.
What Senseible does here
Senseible keeps each source file fingerprinted and linked to the line it supports, so sampled items can be pulled quickly. Third-party verification is done by an independent verifier.
Sources: ISO 14064-3:2019, Specification with guidance for the verification and validation of greenhouse gas statements, ISO 14065:2020, Requirements for bodies validating and verifying environmental information
Last reviewed 2026-10-18.