Direct answer: Historical prices can be compared without an inflation series, but only as dated label observations after matching pack quantities and preserving currency, place, tax context, and source. Do not manufacture an inflation-adjusted figure.
Four terms without the jargon
- Nominal price: the amount printed or displayed at the time.
- Real price: an estimate adjusted to compare purchasing power across dates.
- Inflation series: a dated index used for that adjustment.
- Purchasing power: what money could buy in a particular place and period.
A copyable observation table
| Source | Amount | Currency | Pack quantity | Place/date | Tax context |
|---|---|---|---|---|---|
| Archive A | 18 | Luma | 10 | North Quarter, 2018-06 | Tax treatment not stated |
| Archive B | 40 | Luma | 20 | North Quarter, 2022-06 | Tax-inclusive label |
Luma, the places, amounts, and records are fictional. They must not be indexed as real price facts.
Calculate only what the inputs support
The first synthetic observation normalizes to 36 Luma per 20 units: 18 ÷ 10 × 20. The second is 40 Luma per 20. That permits a limited nominal statement: the displayed per-20 amount in these two records is 36 and 40 Luma. It does not show a real-price increase because no suitable inflation index is supplied and the tax contexts differ.
Comparability checklist
Before comparing, match product quantity, currency definition, geography, observation date, seller or source type, promotion conditions, and whether tax is included. If a currency was redenominated, retain the old unit and use the official conversion rule with its effective date. Never treat two currencies with the same symbol as identical.
Inflation and uncertainty rules
An annual index may be too coarse for a monthly claim. A national index may not answer a local-market question. If a suitable series exists, name it, state the base period, show the formula, and avoid more decimal places than the inputs justify. If the series has gaps or changed methodology, report the limitation or omit the adjusted result.
Tax-exclusive and tax-inclusive labels should remain separate unless an authoritative rate and applicability rule support adjustment. A geographic mismatch cannot be repaired by arithmetic.
The repeatable takeaway
Preserve the historical labels, compare like quantities, disclose every conversion, and do not invent an adjusted number. “Not calculated” is more useful than false precision.
Adult and compliance notice
This is neutral historical-price methodology for adults. It states no real or current product price, offer, stock, legality, affordability, health, or safety conclusion.