HAD Phase 3: pre-test diagnostics (qug_test, stute_test, yatchew_hr_test, composite workflow)

TODO.md

@@ -95,7 +95,11 @@ Deferred items from PR reviews that were not addressed before merge.
 | `HeterogeneousAdoptionDiD`: `weights=` support. Deferred jointly with survey integration. nprobust's `lprobust` has no weight argument so the nonparametric continuous path needs a derivation; the 2SLS mass-point path needs weighted-sandwich parity. | `diff_diff/had.py` | Phase 2a | Medium |
 | `HeterogeneousAdoptionDiD` mass-point: `vcov_type in {"hc2", "hc2_bm"}` raises `NotImplementedError` pending a 2SLS-specific leverage derivation. The OLS leverage `x_i' (X'X)^{-1} x_i` is wrong for 2SLS; the correct finite-sample correction uses `x_i' (Z'X)^{-1} (...) (X'Z)^{-1} x_i`. Needs derivation plus an R / Stata (`ivreg2 small robust`) parity anchor. | `diff_diff/had.py::_fit_mass_point_2sls` | Phase 2a | Medium |
 | `HeterogeneousAdoptionDiD` continuous paths: thread `cluster=` through `bias_corrected_local_linear` (Phase 1c's wrapper already supports cluster; Phase 2a ignores it with a `UserWarning` on the continuous path to keep scope tight). | `diff_diff/had.py`, `diff_diff/local_linear.py` | Phase 2a | Low |
-| `HeterogeneousAdoptionDiD` Phase 3: `qug_test()`, `stute_test()`, `yatchew_hr_test()` pre-test diagnostics (paper Section 3.3). Composite helper `did_had_pretest_workflow()`. Not part of Phase 2a scope. | `diff_diff/had.py`, new module | Phase 2a | Medium |
+| `HeterogeneousAdoptionDiD` Phase 3 joint Equation 18 cross-horizon Stute test: paper's step 2 of the four-step pre-testing workflow tests joint pre-trends via a stacked-residual CvM across pre-period placebos. Phase 3 shipped the single-horizon Stute in `did_had_pretest_workflow()`; the joint variant needs the exact stacked-residual formula extracted from the paper PDF (not reproduced in `dechaisemartin-2026-review.md`). Follow-up patch. | `diff_diff/had_pretests.py` | Phase 3 | Medium |
+| `HeterogeneousAdoptionDiD` Phase 3 Stute performance: Appendix D vectorized matrix form replaces the per-iteration OLS refit with a single precomputed `M = I - X(X'X)^{-1}X'` applied to `eps * eta`. Functionally identical, ~2x faster. Shipped literal-refit form in Phase 3 to match paper text and keep reviewer surface small. | `diff_diff/had_pretests.py::stute_test` | Phase 3 | Low |
+| `HeterogeneousAdoptionDiD` Phase 3 R-parity: Phase 3 ships coverage-rate validation on synthetic DGPs (not tight point parity against `chaisemartin::stute_test` / `yatchew_test`). Tight numerical parity requires aligning bootstrap seed semantics and `B` across numpy/R and is deferred. | `tests/test_had_pretests.py` | Phase 3 | Low |
+| `HeterogeneousAdoptionDiD` Phase 3 nprobust bandwidth for Stute: some Stute variants on continuous regressors use nprobust-style optimal bandwidth selection. Phase 3 uses OLS residuals from a 2-parameter linear fit (no bandwidth selection). nprobust integration is a future enhancement; not in paper scope. | `diff_diff/had_pretests.py::stute_test` | Phase 3 | Low |
+| `HeterogeneousAdoptionDiD` Phase 3 multi-period workflow dispatch: `did_had_pretest_workflow` accepts two-period overall-path panels only. Multi-period users pre-slice to `(F-1, F)` before calling. A follow-up could add `aggregate="event_study"`-like dispatch for joint pre-trend diagnostics alongside Equation 18. | `diff_diff/had_pretests.py::did_had_pretest_workflow` | Phase 3 | Low |
 | `HeterogeneousAdoptionDiD` Phase 4: Pierce-Schott (2016) replication harness; reproduce paper Figure 2 values and Table 1 coverage rates. | `benchmarks/`, `tests/` | Phase 2a | Low |
 | `HeterogeneousAdoptionDiD` Phase 5: `practitioner_next_steps()` integration, tutorial notebook, and `llms.txt` updates (preserving UTF-8 fingerprint). | `diff_diff/practitioner.py`, `tutorials/`, `diff_diff/guides/` | Phase 2a | Low |
 | `HeterogeneousAdoptionDiD` time-varying dose on event study: Phase 2b REJECTS panels where `D_{g,t}` varies within a unit for `t >= F` (the aggregation uses `D_{g, F}` as the single regressor for all horizons, paper Appendix B.2 constant-dose convention). A follow-up PR could add a time-varying-dose estimator for these panels; current behavior is front-door rejection with a redirect to `ChaisemartinDHaultfoeuille`. | `diff_diff/had.py::_validate_had_panel_event_study` | Phase 2b | Low |

diff_diff/__init__.py

@@ -60,6 +60,16 @@
     HeterogeneousAdoptionDiDEventStudyResults,
     HeterogeneousAdoptionDiDResults,
 )
+from diff_diff.had_pretests import (
+    HADPretestReport,
+    QUGTestResults,
+    StuteTestResults,
+    YatchewTestResults,
+    did_had_pretest_workflow,
+    qug_test,
+    stute_test,
+    yatchew_hr_test,
+)
 from diff_diff.estimators import (
     DifferenceInDifferences,
     MultiPeriodDiD,
@@ -442,6 +452,15 @@
     "HeterogeneousAdoptionDiDResults",
     "HeterogeneousAdoptionDiDEventStudyResults",
     "HAD",
+    # HeterogeneousAdoptionDiD pre-test diagnostics (Phase 3)
+    "qug_test",
+    "stute_test",
+    "yatchew_hr_test",
+    "did_had_pretest_workflow",
+    "QUGTestResults",
+    "StuteTestResults",
+    "YatchewTestResults",
+    "HADPretestReport",
     # Datasets
     "load_card_krueger",
     "load_castle_doctrine",