diff --git a/CHANGELOG.md b/CHANGELOG.md index 39888e0d..896aecb9 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0 ## [Unreleased] ### Added +- **`MultiPeriodDiD(absorb=..., vcov_type in {"hc2", "hc2_bm"})` now supported** (`diff_diff/estimators.py:1476`). Mirrors the DiD-absorb auto-route shipped earlier in this release: when `absorb=` is paired with `vcov_type in {"hc2","hc2_bm"}`, `MultiPeriodDiD.fit()` promotes the absorb columns to `fixed_effects=` internally so the existing full-dummy-design code path computes the algebraically correct vcov on the event-study design (`treated + period_X dummies + treated:period_X interactions + factor(unit)`). Verified at ~1e-10 vs `lm() + sandwich::vcovHC(type="HC2")` and `lm() + clubSandwich::vcovCR(cluster=1:n, type="CR2")` on a 5-cohort × 5-period event-study fixture (new `tests/test_estimators_vcov_type.py::TestMPDAbsorbedFERParity` against `benchmarks/data/clubsandwich_cr2_golden.json` scenario `mpd_absorbed_fe_did`). HC1/CR1 paths on `absorb=` are unchanged (no leverage term). `TwoWayFixedEffects(vcov_type in {"hc2","hc2_bm"})` rejection remains as a follow-up (different fit-path structure — no `fixed_effects=` equivalent inside TWFE). **Behavioral note (full `MultiPeriodDiDResults` surface change under auto-route):** under the auto-route, the entire returned `MultiPeriodDiDResults` reflects the full-dummy fit rather than the within-transformed fit — `result.coefficients`, `result.vcov`, `result.residuals`, `result.fitted_values`, `result.r_squared` all include the FE-dummy entries / un-demeaned values. `result.period_effects[t].effect` / `.se` / `.p_value` / `.conf_int` and `result.avg_att` / `.avg_se` are invariant to this routing (FWL guarantee). MPD requires a time-invariant ever-treated indicator that lies in the span of the intercept and the post-auto-route unit FE dummies (the exact alias depends on the omitted FE reference category under `pd.get_dummies(drop_first=True)`, not just on "the sum of treated-cohort unit dummies"), so `solve_ols` drops one column from that collinear set under R-style rank-deficiency handling. Which specific column is dropped is pivot-order and dummy-coding dependent (in the shipped parity fixture it is a never-treated unit dummy, not the `treated` main effect itself). The per-period interaction coefficients (`treated:period_X`) and `avg_att` are identified and invariant to that choice; parity tests target those rather than the `treated` main effect. **Survey-design scope (replicate weights):** when `survey_design=` uses replicate weights, the auto-route short-circuits the absorb-refit branch at `estimators.py:1693` and routes through the standard `compute_replicate_vcov` path on the fixed full-dummy design — correct because the design does not depend on replicate weights so no per-replicate refit is needed. **Redundant time-FE skip:** when the routed (or directly-supplied) `fixed_effects` list contains the `time` column, MPD silently skips emitting `