Why This Number

Overview

When you review or hand off a model, the hardest question is “why is this number what it is?” Why This Number answers it by reading the workbook’s proof ledger and walking the selected cell’s precedents, then reporting — per node — whether each input is bound to a verified source and whether each formula has been signed by a reviewer.

Where to find it

On the ModelXcel Pro tab, in the Proof group, click Why This Number. Keytip path: Alt, X, then W N. There is no default keyboard shortcut for this command.

Step by step

1. Select the cell whose value you want to justify.
2. On the ModelXcel Pro tab, in the Proof group, click Why This Number.
3. ModelxcelPro reads the cell’s bindings and last signed formula from the proof ledger and walks its precedents.
4. Review the per-node trust verdict for the result and its inputs.

Expected output

A panel showing the selected cell’s source bindings, its last signed formula, and a trust verdict for each node. Verdicts are:

• Bound — an input pinned to an external source whose snapshot still matches.
• Signed — a reviewer has attested this cell, or it sits inside a signed block whose formulas still match.
• Drifted — a bound source has changed since it was bound and the binding needs review.
• Tampered — the formula or value diverged from the bound/signed snapshot, or a signed block was broken.
• Unbound — a provenance record exists but there is no active binding.
• No record — the cell has not been bound or signed yet.

The verdicts are read from the proof ledger; nothing is written.

Example

A Revenue (Y1) input is bound to a source workbook with Bind Source, and the revenue formula row is signed as a block. Running Why This Number on a downstream total shows the binding (with its snapshot time) and a Signed verdict for the formula. If someone later overwrites the bound input, the verdict flips to Tampered until the model is re-verified.

Caveats & limitations

• It reports status — it does not create bindings or signatures. Use Bind Source / Bind Vector and Sign Cell / Sign Block first.
• No record means the cell simply hasn’t been bound or signed yet — not that anything is wrong.
• Verdicts reflect the ledger’s last known state. After editing inputs or formulas, run Verify Workbook to refresh drift detection.

Safety, undo & recovery

Why This Number is read-only. It does not modify the workbook or append to the proof ledger, so there is nothing to undo.

File artifacts

Reads the workbook’s proof ledger, which is stored inside the workbook (a hidden CustomXMLPart) and mirrored to a .mxproof sidecar. Why This Number is read-only: it does not create or modify any files.

Works with

• Bind Source / Bind Vector — create the source bindings reported here.
• Sign Cell / Sign Block — create the reviewer signatures reported here.
• Verify Workbook — refreshes drift state so verdicts stay accurate after edits.
• Coverage — aggregates how much of the workbook is bound and signed. See the Proof & Trust overview.

Troubleshooting