How it works

Scoring

Each item is scored on its own: 100 × max(0, 1 − error ÷ 2), where error = |guess − price| ÷ price. An exact guess earns 100; being off by 100% or more earns 0 (from 200% upward when overestimating). Your receipt score is the average of item scores. Unanswered items score 0. Items with no price for that era are not scored.

Ties between equal scores are broken by lower average error, then earlier submission.

The daily receipt

Resets at 00:00 UTC. Everyone gets the same basket, place, year and answer key. Only receipts that pass our data-quality checks (method documented, source named, uncertainty range under 80% wide) enter daily play. Your first daily attempt is the one that counts.

Where prices come from

Every price is labeled as verified (an exact cited historical price), interpolated (between two observed prices) or model estimate (anchored to national average-price or food-index data). Right now all prices in the game are model estimates awaiting editorial review against the named statistical series — we say so on every result.

Estimates use: price × (target-year index ÷ reference-year index), preferring food-specific indices over general inflation. Long-range estimates (like 1891) carry wide uncertainty and are kept out of daily play.

Prices are always shown in the local currency of that time. Old currencies are converted with documented official rates (e.g. UK pre-1971: £1 = 240 old pence). We never compare amounts across currencies as if they were equal.

Rankings

Daily scores are posted to a global leaderboard stored on our server. Only your first daily attempt counts, and scores are re-graded server-side — a client can never submit a score it didn't earn. Your personal history is also stored on your device.

Editorial review & corrections

Every price carries a review status. Unreviewed means the estimate has not yet been checked against an exact cited observation in the named statistical series; reviewed means an editor attached that citation. We never label a price "verified" before that check happens, and we don't claim fact-checking that didn't occur.

Spotted a price that looks wrong? The research pages show each item's source series and uncertainty range — if you have a better citation, open the source link on that page and compare. Corrections are applied to the dataset and change the review status only after an editor verifies them.