Model audit

Reviewing a spreadsheet for correctness before its output is trusted.

The recurring errors are consistent across every industry: a hardcoded number inside a formula, a range that stopped short of the last row, a copied formula with the wrong anchoring, a sign convention flipped halfway down a column, and an assumption entered twice in different places.

The defences are structural. One colour for inputs, formulas everywhere else, no constants inside formulas, a check row that must sum to zero, and someone else reading it.

Every model has an error. The question is whether it changes the answer.