# Methodology

## Universe and spine
The 978 securities and 994 membership periods of Historical Cohort Release 1.1 (reconstructed S&P 500 membership from SEC-filed index-fund holdings anchors and an evidenced event ledger). The crosswalk's day-coverage per security equals the security's Cohort membership days exactly (verified in the release certification: 3,851,484 member-days, no gaps, no overlaps).

## Evidence
Only original SEC EDGAR documents: Form 4 issuer filings, periodic-report covers (10-K/10-Q), SC 13D/G, Form 25, 8-K, fails-to-deliver files joined by name, and SEC index files. No exchange data, no commercial price or reference feed, no vendor identifier. Source URLs and content hashes are in the provenance file. CUSIP values were used only as internal join keys during research and are excluded from this release by an automated check.

## Tiers
| Tier | Meaning | Detail |
|---|---|---|
| `A` | Documented single-day point | A single day documented in an original SEC filing (and, for one row, a fund holding). Nine days in total. |
| `B` | SEC-evidenced interval | An interval supported by original SEC filings (mostly Form 4 issuer symbol chains and periodic-report covers) that bracket it. Issuer identity and symbol as filed; not exchange confirmation. |
| `B2_FTD` | SEC ownership filings plus fails-to-deliver bounded | Intervals admitted under a frozen secondary rule combining SEC 13D/G ownership filings and SEC fails-to-deliver observations. Weaker than B; kept separate. |
| `INFERRED` | Inferred between two SEC anchors | A gap bracketed by two SEC-observed B anchors with the same issuer CIK and ticker, 366 days or shorter. The gap itself is not witnessed by any filing. Unidentified corporate actions or class changes may exist inside it. |
| `C_UNRESOLVED` | Unresolved | Membership days for which no accepted identity evidence exists. ticker and issuer_cik are blank. Present deliberately so coverage can be measured, not hidden. |

## Rules that matter
- Segments are half-open and exactly partition each security's membership days.
- A ticker or CIK is only recorded where an accepted filing supports it; where none exists the day is C_UNRESOLVED, never filled.
- Ticker changes were checked against effective dates; six material errors found in older research editions were corrected (see EXCEPTIONS.csv).
- Where two tickers alternate for one security (Applera CRA/ABI), the ambiguity is preserved and listed as an exception rather than resolved by choice.
- Share class is recorded as evidence found (`class_evidence`), not as a verified listing class.

## What this release does not do
No exchange-certified identity, no trading continuity, no price join, no corporate-action rights, no point-in-time visibility timestamps, no coverage beyond 2025-09-30, no FIGI/CUSIP/ISIN. Identity work for 2026 exists internally and is not part of this release.

## Reproducibility
Built by `scripts/commercial/crosswalk/build.py` from a frozen internal baseline (input SHA-256 `e12c5e02bcab51d7dec229b22d24bfa8b74a9df6c439541e32aaa5c4c0ab9363`) and the Cohort 1.1 membership file. The build is byte-deterministic.