A model is a living document edited by many hands over months; without discipline it rots into a file no one dares change. Two practices keep it alive.
Version control (the discipline, since Excel has no git). Name files with a dated, versioned convention: Project_Model_v1.4_2026-07-20.xlsx, never Model_final_FINAL_v2_real.xlsx. Keep a change log sheet inside the workbook: date, author, what changed, why. Freeze the input block against accidental edits. Review ▸ Protect Sheet (Alt, R, P, S) with only the blue input cells unlocked (select inputs ▸ Ctrl+1 ▸ Protection ▸ untick Locked before protecting), so a user can change assumptions but not overwrite a formula. Snapshot before any major surgery (Save As a new version), so you can always roll back. On teams, the model lives in one controlled location (SharePoint/OneDrive with version history), not as five email attachments.
What does a peer reviewer check in a financial model?
Peer review (the discipline that catches what you can't see). No modeller can fully audit their own model, you read what you meant to write. A second pair of eyes runs a checklist: are the checks all green? Is every input blue and in one block? Does any formula in a projection row disagree with its neighbours (Row differences)? Are there red external links, and are they intentional? Any hardcodes buried in formulas (Show Formulas scan)? Does iterative calc need to be on, and is the circularity contained by a breaker? This is the human layer of the FAST standard. Flexible, Appropriate, Structured, Transparent, the codified modelling philosophy that says a model should be transparent enough that a stranger can follow it, structured enough that its logic flows predictably, appropriate to the decision it serves, and flexible enough to change without breaking. The checks row, the colour code, the consistent-formula rule, and the one-role-per-sheet layout are all FAST principles made concrete.
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What is a LAMBDA library in Excel?
LAMBDA libraries: reusable logic, defined once. M365 lets you package a calculation as a named LAMBDA function (Formulas ▸ Name Manager, Ctrl+F3 ▸ New), so a check you use everywhere is written once and reused by name. Define, say, BALCHECK = LAMBDA(assets, liabs, equity, ROUND(assets-(liabs+equity),0)) and every balance check on every sheet becomes =BALCHECK(...), one definition to audit, one place to fix. A small workbook-level library of such functions (a MARGINAL_TAX, a MID_YEAR_DF discount factor, your BALCHECK) is how professional modellers stop re-deriving the same logic and start building transparent, reusable model components, the exact spirit of the FAST "structured and transparent" rules, and the bridge to the capstone in EX7.02.