Model documentation a successor can actually use

The decision in brief

Maintain a concise model summary plus a reproducible technical specification and dated evidence archive. Document information timing, missing-data behavior, reference-price conventions, version boundaries, and responsible owners. Test the documentation by asking someone other than the author to reconstruct an output without informal instructions.

Begin with purpose, authority, and exclusions

The front page should state the model's objective, covered universe, horizon, intended readers, and permitted uses. Describe what the output means and what it does not mean. A security ranking is not necessarily a position-size instruction. A risk estimate is not a guarantee of an exit price. Identify whether the model informs research, publishes recommendations, or participates in an authorized investment process.

Name the model owner, reviewer, operational contact, and approval reference. Include known limitations where users will see them, not only in a technical appendix. CFA Institute's communication guidance discusses explaining investment processes, significant risks, and material changes. Treat those topics as useful documentation prompts, while keeping the office's particular obligations and professional-standard applicability separate.

Define inputs by availability and meaning

For each input, document the vendor or original source, field definition, units, currency, calendar, economic date, publication lag, and revision treatment. Specify historical universe membership and identifier mapping. A field called price or earnings is insufficient: adjusted and unadjusted prices, total returns, and different earnings definitions can produce different model behavior even when file names look similar.

MSCI's methodology resources distinguish calculation rules, fundamental data, and corporate events, illustrating the value of separate specifications. AQR's data-library notes distinguish original research series from updated series. The office's own record should state exactly which version was used. Preserve permitted snapshots or references sufficient for reconstruction, subject to the actual data license and retention requirements.

Keep document layers aligned without duplicating authority

This original comparison assigns different questions to different records. Link them through a shared version identifier rather than copying rules into several uncontrolled documents. If two records disagree, define which is authoritative and prevent reliance until the inconsistency is resolved. A readable summary should simplify presentation without changing the method.

Record	Primary reader	Essential content
Model summary	Committee and decision owner	Purpose, permitted use, assumptions, limitations
Technical specification	Analyst and independent reviewer	Inputs, timing, transformations, rules, edge cases
Evidence archive	Validation and audit reviewer	Frozen studies, dated publications, test findings
Operating procedure	Service and implementation owner	Release schedule, missing-input response, escalation
Change record	All affected users	Reason, effective date, approvals, evidence impact

Specify ambiguous branches before they occur

Write down how ties, missing observations, stale prices, market closures, instrument substitutions, and discontinued coverage are handled. Specify whether a threshold comparison is strict or inclusive and when a new observation becomes eligible. For research performance, freeze the hypothetical entry and exit reference policies and observation windows. Do not let an implementer choose whichever convention makes a historical result look best.

In a hypothetical ranking model, two securities tie at the inclusion cutoff and one has a missing liquidity field. The specification should determine whether the tie is resolved by an explicit rule, both are excluded, or the release is withheld. The reviewer should not have to ask the author for an improvised answer. Any later policy change creates a version boundary with a stated effective date.

Link evidence to the exact model version

Attach study periods, data snapshots, candidate-search history, validation findings, and unresolved limitations to the version they assess. Record a study's creation date separately from the historical dates it analyzes. Archive original live research publications independently of later corrected analyses. Keep order and execution evidence in its own record so a research reference cannot be mistaken for an account transaction.

In a hypothetical correction, a vendor revises a past observation and the recalculated signal changes. Retain the original release, create the corrected analysis, and explain which decisions were potentially affected. Do not overwrite the publication archive. CFA Institute's record-retention guidance supports preserving the basis for analysis, but the office must reconcile its own retention obligations with contractual rights to the source material.

Test handover and maintain change discipline

Ask an independent analyst to reconstruct a selected output using only the approved documentation and permitted artifacts. Log every clarification the author supplies. Those clarifications reveal documentation gaps even if the numerical result eventually matches. Software correctness tests and a successful handover support reproducibility; they do not independently validate investment skill or prove that an operational service has run as intended.

Failure cases include an undocumented dependency, a summary that describes a different model version, and a change log that omits altered exclusions. Schedule review after material data, methodology, or ownership changes. Retire obsolete versions explicitly while preserving historical identity. A successor should be able to answer what was authorized, what was published, and what was actually executed without reconstructing the process from personal messages or memory.

Decision points

Connect purpose, technical rules, evidence, and operations through one model identity.
Document timing and edge cases before an implementer must improvise.
Verify the documentation through reconstruction and preserve historical version boundaries.

Sources and further reading

The linked sources provide conceptual background. The worked examples and planning assumptions on this page are this publication's own illustrations.