Data quality in greenhouse gas (GHG) reporting describes how reliably each emissions source has been measured. The GHG Protocol explicitly requires companies to assess and report data quality, because two footprints with the same headline number can be very different in defensibility depending on the data behind them.
How it is tiered
- Tier 1 (highest quality): primary activity data from direct measurement — metered kWh, fuel receipts in litres, weighed waste records, supplier-confirmed quantities.
- Tier 2: primary data with some estimation — partial-year data extrapolated to a full year, distance-based vehicle calculations where fuel data is unavailable, or estimated splits of waste streams.
- Tier 3 (lowest quality): secondary data or modelled estimates — industry averages, national statistics, or spend-based proxies. Used only as a last resort.
Why it matters
A footprint where 80%+ of emissions are calculated from Tier 1 data is materially more credible than one dominated by Tier 3 estimates. Auditors will trace samples back to source data; procurement reviewers will look for evidence of progress in moving sources up the tier hierarchy year-on-year.
Reporting data quality also makes the footprint defensible. Where Tier 3 estimates are unavoidable, documenting them transparently is far more credible than burying them in a single headline figure.
A practical example
A company calculates its Scope 1 fleet emissions from actual diesel litres on fuel-card statements (Tier 1). It calculates Scope 2 from monthly metered kWh (Tier 1). For one supplier in Category 1, it uses spend × an industry-average factor because the supplier cannot provide tonnage (Tier 3). Each is reported alongside its tier rating.
We document data quality for every source in every report. See our tiered approach on the methodology page.