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24 October 2026 · Updated at 07:47 GMT FR EN ES Member area Log in
Market

Methodology

Collection, checking, calculation and publication: the detailed method that produces the OIMAP art market statistics.

Updated : October 28, 2026· 3 min read

A method published before it is applied

Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748)
Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748)

All the Observatory's statistics rest on a written method, submitted each year to the scientific council. This page summarises its steps and limits.

The data come from standardised declarations sent by approved operators: auction houses, galleries, platforms and administrations. Each sale is attached to the work's OIMAP number where one exists.

The calculation of the index rests on the repeat-sales method, which compares the successive prices of the same work.

  • Sources
  • Check
  • Calculation
  • Revision

The method in figures

1,120Declaring auction houses
41,800Sale pairs
±0.6Point of margin of error
3 monthsRevision period

Source: 2024 methodological note.

The five steps of the method

  1. Collection

    Monthly declarations standardised by approved operators.

  2. Matching

    Attachment of each sale to the work's OIMAP number.

  3. Check

    Detection of duplicates, outliers and linked sales.

  4. Calculation

    Regression on sale pairs, weighting by segment.

  5. Publication

    Published with margin of error and method note.

Why repeat sales

Compare a work with itself.

A work is unique. Comparing the average price from one year to the next is misleading, because the works sold are not the same. The repeat-sales method gets around this problem by comparing a work with itself.

If a painting is sold for €120,000 in 2018 and then €156,000 in 2024, the 30% change enters the calculation. By aggregating tens of thousands of cases, an average price change at constant quality is obtained.

Peregrine Falcons (Duck Hawks), John James Audubon (c. 1827)
Peregrine Falcons (Duck Hawks), John James Audubon (c. 1827)
Seal with Two Qilin Playing (1849)
Seal with Two Qilin Playing (1849)
Flowers and Trees of the Four Seasons, Watanabe Shikō (early-mid 1700s)
Flowers and Trees of the Four Seasons, Watanabe Shikō (early-mid 1700s)

The data sources

Where the declarations come from
SourceShareCheck applied
Approved auction houses58%Reconciliation with the minutes of sales
Galleries and dealers24%Invoice and certificate
Digital platforms12%Transaction log
Administrations6%Customs declarations

Reasons for excluding a sale pair

Work restored between two sales34%
Attribution changed27%
Linked sale between related parties21%
Interval of less than six months18%

Share of pairs excluded in 2024.

Evolution of the method

  1. First method

    Simple repeat sales.

  2. Weighting

    Weight by segment.

  3. Constant euros

    Correction of exchange rates.

  4. Control of linked sales

    Exclusion of transactions between related parties.

  5. Matching by OIMAP number

    Greater reliability of pairs.

Limits of the method

Repeat sales or hedonic prices

View of Venice, Jacopo de' Barbari (1500)

Repeat sales

Method adopted
  • Compares a work with itself
  • Few variables to model
  • Excludes works sold only once

Robust for the global index.

Telescoping basket (1800's)

Hedonic prices

Complementary method
  • Models characteristics
  • Uses all sales
  • Depends on the model chosen

Used for sector studies.

Checks applied to each declaration

Carved Lacquer Scroll Box (1736–95)
Carved Lacquer Scroll Box (1736–95)
Leaf from a Psalter and Prayerbook: Ornamental Border with Carnations, a Thistle, and a Cook Ladling Soup (verso) (c. 1524)
Leaf from a Psalter and Prayerbook: Ornamental Border with Carnations, a Thistle, and a Cook Ladling Soup (verso) (c. 1524)
Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748)
Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748)

A member of the scientific council

Medal: In Commemoration of the Fiftieth Anniversary - The Warner and Swasey Company, Victor D. Brenner (1930)
« A good method is recognised by what it says about its own limits. »
Prof. Ingrid Sørensen, statistician, University of Oslo

Vocabulary of the method

Repeat sale
Resale of the same work at a different price.
Outlier
Price very far from the average, to be checked.
Linked sale
Transaction between related parties.
Constant euros
Euros adjusted for inflation.

Frequently asked questions

Are private sales included?

Yes, when they are declared by an approved operator.

Who checks the calculations?

The scientific council, which examines the method every October.

Can I redo the calculation myself?

Yes, the databases contain the anonymised pairs.

What happens in case of error?

A public correction is published, with the revised series.

Useful documents

Contacts

The Pyramid of Cestius, Rome, Michiel van Overbeek (c. 1650–80)
The Pyramid of Cestius, Rome, Michiel van Overbeek (c. 1650–80)
OIMAP ObservatoryParis — Headquarters+33 1 00 00 01 00observatoire@oimap.exampleMonday to Friday, 9 am – 5 pm

Download the data

Document Box (Ryōshibako) with Chrysanthemum and Plum (lid) (late 1800s-early 1900s)

Reproduce the calculations from the open databases.

Databases
Image credits (16)
  • An Indigo Factory, Allahabad, Louis-Théophile Marie Rousselet (1865–68, published 1869–70) — Cleveland Museum of Art, CC0 1.0
  • Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748) — Cleveland Museum of Art, CC0 1.0
  • Peregrine Falcons (Duck Hawks), John James Audubon (c. 1827) — Cleveland Museum of Art, CC0 1.0
  • Seal with Two Qilin Playing (1849) — Cleveland Museum of Art, CC0 1.0
  • Flowers and Trees of the Four Seasons, Watanabe Shikō (early-mid 1700s) — Cleveland Museum of Art, CC0 1.0
  • View of Venice, Jacopo de' Barbari (1500) — Cleveland Museum of Art, CC0 1.0
  • Telescoping basket (1800's) — Cleveland Museum of Art, CC0 1.0
  • Carved Lacquer Scroll Box (1736–95) — Cleveland Museum of Art, CC0 1.0
  • Leaf from a Psalter and Prayerbook: Ornamental Border with Carnations, a Thistle, and a Cook Ladling Soup (verso) (c. 1524) — Cleveland Museum of Art, CC0 1.0
  • Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748) — Cleveland Museum of Art, CC0 1.0
  • Medal: In Commemoration of the Fiftieth Anniversary - The Warner and Swasey Company, Victor D. Brenner (1930) — Cleveland Museum of Art, CC0 1.0
  • The Pyramid of Cestius, Rome, Michiel van Overbeek (c. 1650–80) — Cleveland Museum of Art, CC0 1.0
  • Annunciation to the Shepherds: Leaf from a Book of Hours (1 of 6 Excised Leaves), Henri d'Orquevaulx (c. 1420–30) — Cleveland Museum of Art, CC0 1.0
  • Nuova pianta di Roma data in luce da Giambattista Nolli l’anno MDCCXLVII, known as La Pianta Grande di Roma, Giovanni Battista Nolli (1748) — Cleveland Museum of Art, CC0 1.0
  • Illuminated Opening Page Titled Laila and Majnun from a Khamsa (Quintet) of Nizami of Ganja, Ja'far Baisunghuri (835 AH/1431–32 CE) — The Metropolitan Museum of Art, CC0 1.0
  • Document Box (Ryōshibako) with Chrysanthemum and Plum (lid) (late 1800s-early 1900s) — Cleveland Museum of Art, CC0 1.0
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