How we measure accuracy — and why we will not claim a perfect score

A converter that advertises a field-level accuracy figure — 99% or 99.9% — is not lying, but it is answering a different question from the one you are asking, and the gap between the two is larger than it looks.

Field accuracy is not statement accuracy

A transaction is about five fields: date, description, money in, money out, balance. A fifty-transaction statement is therefore around 250 fields, and the chance of the whole statement being clean is the per-field rate raised to the power of the field count. At 99.9% per field, only about 78% of fifty-transaction statements come back with no errors at all. At 99%, it is 8%.

What we report instead

A bank statement carries its own answer key: the running balance column. So rather than quoting a number you cannot check, we re-derive every row from it and report one of three states per row — verified, flagged, or not checkable — along with which whole-statement checks passed and which could not run.

• Verified — the change in the printed balance equals this row’s amount, exactly.
• Flagged — it does not, and we show you the row and the discrepancy.
• Not checkable — payee text, which arithmetic cannot validate. We say so rather than folding it into a percentage.

Why the per-row check is the one that matters

While building this we ran the checker over deliberately corrupted data and found something worth knowing: two equal and opposite errors made the closing balance and the overall total agree perfectly. The extra five pounds entered the ledger and left it again, so both endpoint checks passed on data that was wrong. Only the row-by-row check caught it. A tool that checks just the closing balance will hand you a green light over corrupt figures.

The per-bank numbers

There are none yet. No real bank statement has been through this tool under test, so no bank page carries an accuracy figure. When real statements have been run, a bank page will show how many, what share came back with every row verified, and where that record is. If a later layout breaks something, the worse figure goes up in its place.

What we cannot check

Descriptions. Arithmetic can prove an amount and it can prove a date sits in sequence, but nothing can prove a payee name was read correctly. If the payee text matters for your purpose — categorisation, an audit trail, a legal filing — read it. And on any statement with no running balance column at all, the strongest check simply cannot run, which we say on the results screen rather than quietly downgrading what “verified” means.