Methodology · External correlations
Cross-validating the Index against external benchmarks
The Artmetria Index is built on its own corpus and methodology. To check that the displayed series moves in a way consistent with established art-market benchmarks, we compare it against published yearly values from third-party indices on overlapping time windows. That external dataset is still being assembled — until it is complete, the internal consistency audit below carries the validation.
None of the comparisons below adjust the Artmetria Index. They are a sanity check — either the directional motion agrees and the methodology earns credibility, or it doesn't and the divergence becomes a question worth investigating.
How the correlation is computed
For each external source / vertical pair, we aggregate the Artmetria daily snapshots to a yearly mean, then join with the published external values on the year key. The audit reports both Pearson (linear) and Spearman (monotonic-rank) correlation coefficients.
Pearson is sensitive to the absolute level alignment ; Spearman is robust to scale and only measures whether the two series move in the same direction. For cross-validation a strong Spearman with a weak Pearson is acceptable — that just means the two indices use different base levels.
Reference points are transcribed manually from public sources (annual reports, press releases, industry surveys). Each row carries a source_url for traceability.
External index reference points on file
The reference table (index_external_references) is currently empty. We are in the process of transcribing the most recent yearly values published by:
- Sotheby's Mei Moses — repeat-sale art market index, yearly since 2002, sub-indices by period
- Artprice100& Artprice global — auction-weighted artist index, yearly with sector breakdowns
- The Art Market Report (UBS / Art Basel) — annual auction turnover by segment
- HAGI Top 50 — collectible car index, yearly
- Liv-ex Fine Wine 100 — fine wine index, monthly
Each row will cite the public document it was transcribed from. The audit comparison auto-renders here when the table is populated.
Internal consistency results
While we build out the external reference dataset, we publish the internal consistency audit run by the same script. Each vertical's yearly index series is compared against its own 3-year centered moving average :
- Pearson vs smoothed> 0.95 → the index is internally stable (variation reflects market motion, not noise)
- YoY volatility typically 10–30 % for art-market indices — higher values are expected on more speculative segments (e.g. Contemporary Art)
- % high or medium confidence snapshots — fraction of the 18-year history where the sample size exceeds the 200-lot threshold
Most verticals show Pearson above ~0.95 and YoY volatility within the 10–30 % band; the more speculative segments (e.g. Contemporary Art) run higher, and thin sub-indices such as the 19th-Century one carry insufficient baseline coverage (values derived from sparse historical lots).