A grocery price log with the unit price computed from the pack: barcode identity, pack quantity, shelf price and promotion flag per observation

Goal

Build a price series per product that survives pack-size changes, shop changes and promotions, so that a later comparison sets the same product and quantity side by side rather than a remembered number.

Prerequisites

A barcode app or the number printed under the barcode; a spreadsheet with one row per observation. EU Directive 98/6/EC defines the unit price as the final price, including all taxes, for one kilogramme, one litre, one metre, one square metre or one cubic metre of the product, or another unit customary in the member state, and many shelf labels carry such a figure. The protocol nevertheless computes its own unit price from the pack, because label conventions differ between shops and the computation must be reproducible from the row.

Steps

1. Product table: barcode number as key, name as printed, brand, pack quantity as printed with its unit, and the unit chosen for the computed unit price (per kg, per L, per piece). A new pack size is a new product row, linked by a "product family" column.
2. Observation row: date, shop and branch, barcode number, shelf price, whether the price was regular or a marked promotion (with its condition: "2 for", "member price", "clearance"), the price actually paid if it differed, and the shelf label's own unit price, if shown, in a separate column.
3. Compute the unit price as shelf price divided by pack quantity, in the stated unit, in its own column; keep the label's figure beside it and note disagreements.
4. Log a fixed basket on a fixed cadence (for example the first trip of each month) and on any other trip where a basket product is bought; the basket list is in the header and changes only with a dated note.
5. Record "not stocked" rather than skipping a row; a substitute gets its own barcode row.
6. Where the barcode cannot be read, photograph the label and transcribe later, keeping the filename in the row.

Expected result

A per-product series of unit prices with promotion flags, in which a smaller pack at the same shelf price appears as a unit-price step on a new product row rather than as an unchanged price, and shop differences are visible because the shop is a column.

Limits and test basis

Proposed protocol; no price level, trend or inflation figure is claimed and no shop or product is recommended. A barcode number identifies a trade item as its manufacturer defines it; a reformulated product may keep its number and a multipack may carry its own. Loyalty prices depend on conditions the log records but does not evaluate. A personal basket is not a price index.