An expense categorisation log as a method: frozen category list, numbered edge-case rules and a re-coding check

Goal

Make monthly category totals of household spending mean the same thing every month and be explainable to another person, by treating categorisation as a coding task with written rules and a measured consistency, rather than as a judgement made afresh at each receipt.

Prerequisites

A source of records (receipts, statement exports, a payment app's history), a spreadsheet or CSV file, and an hour to write the category list before the first transaction is coded.

Steps

1. Write the category list with a one-sentence definition per category and an explicit "unclassified" category; freeze the list for the observation period (a quarter or a year).
2. One row per transaction: date, amount in minor units or as text with two decimals (not as a floating-point number), currency, payee exactly as printed, category, the identifier of the rule applied, and a free note. A transaction spanning categories becomes several rows that sum to the total, flagged as a split.
3. Whenever a transaction does not fit a definition cleanly, decide, and write the decision as a numbered rule in a separate rule sheet (for example, "R07: a supermarket receipt is groceries unless a single non-food item exceeds half the total, in which case split"). Later transactions cite the rule, not the memory of it.
4. Refunds are rows with a negative amount in the original category and a reference to the original row.
5. Monthly re-coding check: hide the category and rule columns of twenty randomly chosen rows, code them again from payee and note alone, then compare; record the number of disagreements as the month's self-agreement figure and resolve each disagreement with a new or amended rule.
6. Change the category list only at a period boundary, and keep a mapping table from old to new categories; historical totals are recomputed only through that mapping.

Expected result

Per month, a total per category, a count of unclassified rows, a self-agreement figure, and a rule sheet that lets someone else reproduce the totals from the same records.

Limits and test basis

Proposed method; the totals depend on the rules, so two households with different rule sheets are not comparable without the sheets. Currency conversion dates, shared purchases and cash without receipts remain judgement calls that the rule sheet can only make explicit. The method concerns how records are categorised and documented; what to spend, save or change is outside it.