Wholesale price index: methodology

Version 1. The figures for a month are computed once, on the 5th of the next month (Türkiye time), and never change afterwards.

What it measures

What goods change hands for between businesses on Birlesa: for each HS heading (four-digit product group), the price per base unit agreed in deals during the month, as a median with the middle half of prices around it (the 25th and 75th percentiles).

Where the prices come from

• Agreed deals only. A price enters when a buyer accepts a supplier's quote (or the supplier accepts the buyer's counter-offer). Quotes nobody accepted are not used.
• Only with both sides' consent. An organization chooses to contribute in its settings. A deal counts only if both its buyer and its supplier had consented before the deal was agreed. An organization can withdraw at any time; the months computed after that do not use its deals.
• Each quote line separately. A line is placed under an HS heading by the product code of the need it answers (when the need names one heading), or otherwise by its description, when our product taxonomy recognizes it with high confidence. Lines that cannot be placed are left out, never guessed.
• Per base unit. Prices are converted to kilograms, pieces, metres, square metres, litres, pairs or sets (a ton price becomes a kilogram price, a dozen price a piece price). Packaging units (boxes, packs, rolls) cannot be compared and are left out.
• Per currency. Prices are not converted between currencies; each currency is its own series.

Protecting the businesses behind the prices

No single deal or business can be read from what is published.

1. Outliers are left out first. A price further than 3.5 robust deviations from the series' median is dropped (the deviation is the median absolute deviation, scaled to be comparable with a standard deviation). Entry mistakes and unusual one-off deals do not move the figures.
2. Minimum numbers. A series is published only when at least 10 prices remain, from at least 3 different suppliers and 3 different buyers.
3. No dominant party. No supplier and no buyer may account for more than half of a series' prices.
4. No extremes, no counts. We publish the median and the quartiles, never the lowest or highest price. The number of prices is shown only as a band: 10–19, 20–49, 50–99, 100 or more.
5. One level only. Series are published per four-digit heading only. Series never contain one another, so one cannot be subtracted from another to isolate a smaller group.

A series that fails any of these tests is not published that month. It is not merged into a broader one.

What is published

For each month: the HS heading, the base unit, the currency, the median, the 25th and 75th percentiles, the band of the number of prices, and the median's change from the previous month (when both months were published). Read it on the site, or from the API: GET /v1/price-index (published months), /v1/price-index/{YYYY-MM}, /v1/price-index/headings (the headings with prices) and /v1/price-index/headings/{heading}.

Limits

• The index describes deals made on Birlesa by organizations that chose to contribute, not the whole market. Headings with few deals are not published at all.
• Quality, quantity, delivery terms and payment terms differ between deals; the median does not adjust for them.
• A change from one month to the next can come from a different mix of products within a heading as well as from prices.

Changes to this method

Any change gets a new version number, is described here, and applies from the next computed month; published months keep the version they were computed with.