# How many invoices and bills will a GHG verifier sample?

Canonical: https://senseible.earth/answers/how-many-invoices-does-a-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

1. Keep every source document, indexed to the inventory line it supports.
2. Reconcile each source to an independent record, such as fuel to the purchase ledger and electricity to paid bills.
3. Fix known errors before the verification starts and note the corrections.
4. 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.

- [How auditors verify activity data](https://senseible.earth/answers/how-auditors-verify-activity-data)
- [Materiality in GHG verification](https://senseible.earth/answers/what-is-materiality-in-ghg-verification)
- [Limited vs reasonable assurance](https://senseible.earth/answers/limited-vs-reasonable-assurance-emissions)

Sources: [ISO 14064-3:2019, Specification with guidance for the verification and validation of greenhouse gas statements](https://www.iso.org), [ISO 14065:2020, Requirements for bodies validating and verifying environmental information](https://www.iso.org)

Last reviewed 2026-10-18.