Autoza UK Used Car Index — Research Methodology
The Autoza UK Used Car Index will be a quarterly market report derived from dealer listings on autoza.co.uk. This page documents, in advance of the first release, the data-collection process, outlier-filtering rules, primary source cross-references, minimum sample sizes, and the limitations that will be published alongside every figure under CC BY 4.0.
We publish a cohort median only once that cohort holds enough real UK listings to be meaningful. We have not published an index, so no figure on this site should be quoted as an Autoza UK index. What we do publish each month is the Asking-Price Report: what the cars listed on autoza.co.uk were asking, with its method. It describes one marketplace and is not a UK market index, and its method is written out on its own page: the steps below are for a future index and do not apply to it. Live listing aggregates are also available via our market-data API.
Why we publish this
Autoza publishes methodology documentation for three reasons. First, it allows journalists and researchers to assess whether our data is fit for the specific purpose they have in mind — we want citations that are accurate, not just convenient. Second, it enables independent reproduction: anyone with access to the same primary sources should be able to cross-check our headline figures. Third, it is a condition of our CC BY 4.0 licence: re-users who build on our data must be able to document their sources, which requires us to document ours.
A figure published without a documented methodology is an opinion. A figure with a documented methodology and traceable sources is a fact. We intend our data to be used as facts.
The Asking-Price Report: age, electric and hybrid, and city tables
The monthly Asking-Price Report has three further tables: asking prices by age, electric cars and hybrids by age, and asking prices by city. They belong to the report, not to the future index described further down this page, and they follow the report's own method, which each edition prints under "How we calculated this". They appear in editions published after the October 2026 edition; an earlier edition is never rewritten and does not have them.
Asking prices by age
The median asking price, and the range the middle 80% of asking prices sit in, for cars listed on autoza.co.uk in five age bands (0 to 2 years old, 3 to 5 years old, 6 to 8 years old, 9 to 12 years old and 13 or more years old), for all cars and for each of the most-listed models. A car's age is the year the figures were taken minus the year on its advert, in whole calendar years. Each band is a different set of cars, so the table shows what sellers on Autoza ask at each age, not what any one car loses as it ages.
Electric cars and hybrids by age
The same two figures for electric cars and for hybrids (plug-in hybrids included) in each age band. Where the previous edition had the same row and at least 60 cars stand behind it in both months, the table shows how its median moved; the CSV carries every comparison, marked with how far to trust it. Because age is counted in whole calendar years, every January each car moves up a year, so no change is shown for an age band across a year end.
Asking prices by city
The same two figures for the cars listed in each city. A car is placed in a city by the postcode or place name in its location, the way the Autoza city pages place it, and London takes in all of its areas. A city is published only when at least 30 cars stand behind it and no single seller supplies half or more of them, so that no single seller's prices alone decide a city's median. A city that is missing has too few cars or too few sellers to be shown. A city's figure is what cars listed there are asking on Autoza, not that city's market.
Every row in these tables must clear the report's minimum sample of 30 cars; a smaller group is left out, not rounded up. None of them prints how many cars stand behind a row: each row is marked "rough guide" or "firmer" instead. They are asking prices on one marketplace, not sale prices, and not a UK price index.
Data pipeline — step by step
Listings snapshot
On the first working day after each quarter ends, we extract a point-in-time snapshot of all active dealer listings on autoza.co.uk. Only listings with a complete record — make, model, year, mileage, city/region, fuel type, body type and asking price — are included. Private seller listings are excluded from index figures but published separately in the supplementary CSV.
Outlier removal
We apply a 3-sigma rule on price versus mileage clusters per make/model/year combination. Listings more than 3 standard deviations from the cluster centroid are flagged and excluded from median and mean calculations. For low-volume models (fewer than 10 listings in a cluster), we fall back to a national UK used-car price guide (such as Auto Trader Retail Price Index) as an external reference to decide whether extreme prices are genuine or data errors.
Segment classification
Each make/model combination is assigned to one of eight segments: Compact Hatch, Family Saloon/Estate, Compact SUV, Large SUV, Premium Executive, City Car, MPV/People Carrier, and Light Commercial. Segment definitions follow the SMMT/JATO taxonomy used in the SMMT's monthly registration reports, so Autoza segment data can be cross-referenced directly with SMMT figures without re-mapping.
Primary source cross-reference
Segment-share and registration figures are cross-checked against: (1) SMMT monthly new-car registration statistics for the same period — to validate make-level representation; (2) DVLA "Vehicle Licensing Statistics" tables — to anchor the used/new ratio by age cohort; (3) the RAC Fuel Watch monthly fuel price index — for running-cost calculations; (4) DVLA EV registration data — for EV listing validation.
Days-to-sell calculation
Days-to-sell (DTS) is the median number of calendar days between a listing's first-published date and the date it is marked Sold or Archived on autoza.co.uk. Only listings that were active for at least 48 hours are counted (to exclude duplicate or accidental posts). Listings that have not yet sold are excluded from DTS calculations — we do not impute sell time for active stock.
City and regional pricing
City-level price figures are median asking prices per city/region for the snapshot period, restricted to areas with ≥ 50 qualifying listings. Cities with fewer than 50 listings are grouped into a "Remaining areas" bucket to avoid false precision on small samples. Regions follow the standard UK city/region groupings (London, Manchester, Birmingham, Leeds, Glasgow, etc.).
Headline findings review
All headline findings (the statistics featured prominently in the report) are reviewed by the Autoza Research desk before publication. Findings must be reproducible from the published CSV — every headline number must trace to a specific column and filter in the raw data. If a finding cannot be independently reproduced from the dataset, it is flagged and either corrected or removed.
Volume gate
No index is published until the marketplace holds enough UK stock for the figures to mean anything. Two floors apply: a minimum of 10 qualifying listings in any cohort before that cohort is published at all, and a marketplace-wide minimum below which no index is published in any form. The same thresholds already gate the price figures our AI assistant is allowed to quote. The monthly Autoza Used Car Asking-Price Report is published only when the marketplace-wide floor is met, and it reports asking prices on autoza.co.uk — it is not a UK price index. The Asking-Price Report has its own, simpler method, written out on the report page; the steps on this page are for a future index and do not apply to it.
Open publication
When the volume gate is met, the finalised dataset will be published as the canonical HTML report at autoza.co.uk/research/{slug} under CC BY 4.0, alongside a machine-readable open-data release with a full dataset card, per-cohort sample sizes, and a changelog. Publication targets and permanent-identifier arrangements will be documented here at that point rather than promised in advance.
Primary data sources
The following sources will be used in computing the quarterly index. All are publicly accessible. Links are provided to the specific data series used for cross-referencing.
| Source | Role |
|---|---|
| Autoza listings database | Primary source for all price, mileage, fuel type, and days-to-sell figures |
| SMMT monthly statistics | Cross-reference for make/model market share and new-car registration context |
| DVLA — Vehicle Licensing Statistics | New-vehicle registration totals; used as denominator for used/new ratio |
| RAC Fuel Watch fuel price index | Unleaded petrol and diesel pump prices used in running-cost estimates |
| DVLA EV registration data | Cross-reference for EV listing validation and ultra-low-emission model list |
| DVSA Recall Portal | Vehicle safety recall data cited in model-fault guides — not used for index pricing |
Segment definitions
Our eight segments mirror the SMMT/JATO taxonomy, allowing direct comparison with official UK registration data. A model is assigned to exactly one segment based on its manufacturer's primary body/class designation, not trim level.
Open publication — data lineage
When the first index is released it will be published under CC BY 4.0 as a canonical HTML report on autoza.co.uk plus a machine-readable open-data release, cross-linked via sameAs in our structured data so AI search engines, academic indexers, and data catalogues can resolve them as the same canonical dataset.
We are not naming the hosting platforms, dataset URLs, or file manifests here in advance. Publishing a lineage for a dataset that does not yet exist is exactly the kind of claim this page is meant to prevent. The full manifest will be listed here on the day of the first release.
Limitations and scope
- !Autoza figures are asking prices, not transaction prices. What a buyer finally pays can be lower than the advertised price, and Autoza does not hold sale prices, so we do not estimate by how much.
- !Private seller listings are excluded from headline index figures (included only in supplementary CSV). Private prices are structurally different from dealer prices and mixing them distorts the market signal.
- !Autoza data covers Great Britain and Northern Ireland (the UK) only — no Republic of Ireland data is included.
- !Very low-volume makes (fewer than 10 listings in a quarter) may not be representative. Per-make figures for low-volume makes should be treated with caution.
- !Days-to-sell only reflects listings that sold through autoza.co.uk. Cars sold via phone call before the listing was formally updated as Sold are not captured in this metric.
- !This is not a Government or Office for National Statistics dataset. It is proprietary commercial data from a private marketplace. We have no visibility of listings on other platforms.
Corrections policy
If a material error is identified in a published index — a figure that is wrong, an outlier that should have been filtered but wasn't, or a primary source that has revised its figures — we will issue a correction notice on this page and on the affected report page. Correction notices state what was wrong, what the correct figure is, and when the correction was made. Open-data releases will be versioned: corrected datasets are published as new versions with a clear changelog, and the original erroneous version is retained for transparency. Where a figure cannot be corrected because the underlying sample was never sound, we withdraw it rather than restate it.
To report a potential error: info@autoza.co.uk. We aim to investigate and respond within 5 working days.
Access the data
There is no index dataset to download. The monthly Asking-Price Report has a CSV of its figures, and our public market-data API and stats dashboard give real aggregates from current autoza.co.uk listings — both free to use under CC BY 4.0 with attribution to "Autoza UK" and a link.
Press queries and data licensing enquiries: info@autoza.co.uk. Our founder is available for interview on UK motor-industry data topics. Press kit →