Add Triple Difference (DDD) estimator

Implement the TripleDifference estimator following Ortiz-Villavicencio &
Sant'Anna (2025) methodology for DDD designs requiring two treatment
criteria (group and partition).

Features:
- Regression adjustment, IPW, and doubly robust estimation methods
- Proper covariate handling (unlike naive DDD implementations)
- sklearn-like API with fit(), get_params(), set_params()
- Rich results object with cell means, diagnostics, and inference
- Convenience function triple_difference() for quick estimation

Includes:
- Core estimator in diff_diff/triple_diff.py
- 45 comprehensive tests in tests/test_triple_diff.py
- Tutorial notebook (08_triple_diff.ipynb)
- API documentation in docs/api/estimators.rst
- README updates with usage examples and API reference
- Updated CLAUDE.md and ROADMAP.md

Reference: Ortiz-Villavicencio, M., & Sant'Anna, P. H. C. (2025).
"Better Understanding Triple Differences Estimators." arXiv:2505.09942.

Author: claude

CLAUDE.md

@@ -60,6 +60,13 @@ mypy diff_diff
   - Alternative to Callaway-Sant'Anna with different weighting scheme
   - Useful robustness check when both estimators agree
 
+- **`diff_diff/triple_diff.py`** - Triple Difference (DDD) estimator:
+  - `TripleDifference` - Ortiz-Villavicencio & Sant'Anna (2025) estimator for DDD designs
+  - `TripleDifferenceResults` - Results with ATT, SEs, cell means, diagnostics
+  - `triple_difference()` - Convenience function for quick estimation
+  - Regression adjustment, IPW, and doubly robust estimation methods
+  - Proper covariate handling (unlike naive DDD implementations)
+
 - **`diff_diff/bacon.py`** - Goodman-Bacon decomposition for TWFE diagnostics:
   - `BaconDecomposition` - Decompose TWFE into weighted 2x2 comparisons (Goodman-Bacon 2021)
   - `BaconDecompositionResults` - Results with comparison weights and estimates by type
@@ -148,13 +155,15 @@ mypy diff_diff
   - `05_honest_did.ipynb` - Honest DiD sensitivity analysis for parallel trends violations
   - `06_power_analysis.ipynb` - Power analysis for study design, MDE, simulation-based power
   - `07_pretrends_power.ipynb` - Pre-trends power analysis (Roth 2022), MDV, power curves
+  - `08_triple_diff.ipynb` - Triple Difference (DDD) estimation with proper covariate handling
 
 ### Test Structure
 
 Tests mirror the source modules:
 - `tests/test_estimators.py` - Tests for DifferenceInDifferences, TWFE, MultiPeriodDiD, SyntheticDiD
 - `tests/test_staggered.py` - Tests for CallawaySantAnna
 - `tests/test_sun_abraham.py` - Tests for SunAbraham interaction-weighted estimator
+- `tests/test_triple_diff.py` - Tests for Triple Difference (DDD) estimator
 - `tests/test_bacon.py` - Tests for Goodman-Bacon decomposition
 - `tests/test_utils.py` - Tests for parallel trends, robust SE, synthetic weights
 - `tests/test_diagnostics.py` - Tests for placebo tests

README.md

@@ -71,6 +71,7 @@ Signif. codes: '***' 0.001, '**' 0.01, '*' 0.05, '.' 0.1
 - **Panel data support**: Two-way fixed effects estimator for panel designs
 - **Multi-period analysis**: Event-study style DiD with period-specific treatment effects
 - **Staggered adoption**: Callaway-Sant'Anna (2021) and Sun-Abraham (2021) estimators for heterogeneous treatment timing
+- **Triple Difference (DDD)**: Ortiz-Villavicencio & Sant'Anna (2025) estimators with proper covariate handling
 - **Synthetic DiD**: Combined DiD with synthetic control for improved robustness
 - **Event study plots**: Publication-ready visualization of treatment effects
 - **Parallel trends testing**: Multiple methods including equivalence tests
@@ -93,6 +94,8 @@ We provide Jupyter notebook tutorials in `docs/tutorials/`:
 | `04_parallel_trends.ipynb` | Testing parallel trends, equivalence tests, placebo tests, diagnostics |
 | `05_honest_did.ipynb` | Honest DiD sensitivity analysis, bounds, breakdown values, visualization |
 | `06_power_analysis.ipynb` | Power analysis, MDE, sample size calculations, simulation-based power |
+| `07_pretrends_power.ipynb` | Pre-trends power analysis (Roth 2022), MDV, power curves |
+| `08_triple_diff.ipynb` | Triple Difference (DDD) estimation with proper covariate handling |
 
 ## Data Preparation
 
@@ -866,6 +869,77 @@ print(f"Sun-Abraham ATT: {sa_results.overall_att:.3f}")
 # If results differ substantially, investigate heterogeneity
 ```
 
+### Triple Difference (DDD)
+
+Triple Difference (DDD) is used when treatment requires satisfying two criteria: belonging to a treated **group** AND being in an eligible **partition**. The `TripleDifference` class implements the methodology from Ortiz-Villavicencio & Sant'Anna (2025), which correctly handles covariate adjustment (unlike naive implementations).
+
+```python
+from diff_diff import TripleDifference, triple_difference
+
+# Basic usage
+ddd = TripleDifference(estimation_method='dr')  # doubly robust (recommended)
+results = ddd.fit(
+    data,
+    outcome='wages',
+    group='policy_state',       # 1=state enacted policy, 0=control state
+    partition='female',         # 1=women (affected by policy), 0=men
+    time='post'                 # 1=post-policy, 0=pre-policy
+)
+
+# View results
+results.print_summary()
+print(f"ATT: {results.att:.3f} (SE: {results.se:.3f})")
+
+# With covariates (properly incorporated, unlike naive DDD)
+results = ddd.fit(
+    data,
+    outcome='wages',
+    group='policy_state',
+    partition='female',
+    time='post',
+    covariates=['age', 'education', 'experience']
+)
+```
+
+**Estimation methods:**
+
+| Method | Description | When to use |
+|--------|-------------|-------------|
+| `"dr"` | Doubly robust | Recommended. Consistent if either outcome or propensity model is correct |
+| `"reg"` | Regression adjustment | Simple outcome regression with full interactions |
+| `"ipw"` | Inverse probability weighting | When propensity score model is well-specified |
+
+```python
+# Compare estimation methods
+for method in ['reg', 'ipw', 'dr']:
+    est = TripleDifference(estimation_method=method)
+    res = est.fit(data, outcome='y', group='g', partition='p', time='t')
+    print(f"{method}: ATT={res.att:.3f} (SE={res.se:.3f})")
+```
+
+**Convenience function:**
+
+```python
+# One-liner estimation
+results = triple_difference(
+    data,
+    outcome='wages',
+    group='policy_state',
+    partition='female',
+    time='post',
+    covariates=['age', 'education'],
+    estimation_method='dr'
+)
+```
+
+**Why use DDD instead of DiD?**
+
+DDD allows for violations of parallel trends that are:
+- Group-specific (e.g., economic shocks in treatment states)
+- Partition-specific (e.g., trends affecting women everywhere)
+
+As long as these biases are additive, DDD differences them out. The key assumption is that the *differential* trend between eligible and ineligible units would be the same across groups.
+
 ### Event Study Visualization
 
 Create publication-ready event study plots:
@@ -1661,6 +1735,60 @@ SunAbraham(
 | `print_summary(alpha)` | Print summary to stdout |
 | `to_dataframe(level)` | Convert to DataFrame ('event_study' or 'cohort') |
 
+### TripleDifference
+
+```python
+TripleDifference(
+    estimation_method='dr',   # 'dr' (doubly robust), 'reg', or 'ipw'
+    robust=True,              # Use HC1 robust standard errors
+    cluster=None,             # Column for cluster-robust SEs
+    alpha=0.05,               # Significance level for CIs
+    pscore_trim=0.01          # Propensity score trimming threshold
+)
+```
+
+**fit() Parameters:**
+
+| Parameter | Type | Description |
+|-----------|------|-------------|
+| `data` | DataFrame | Input data |
+| `outcome` | str | Outcome variable column name |
+| `group` | str | Group indicator column (0/1): 1=treated group |
+| `partition` | str | Partition/eligibility indicator column (0/1): 1=eligible |
+| `time` | str | Time indicator column (0/1): 1=post-treatment |
+| `covariates` | list | Covariate column names for adjustment |
+
+### TripleDifferenceResults
+
+**Attributes:**
+
+| Attribute | Description |
+|-----------|-------------|
+| `att` | Average Treatment effect on the Treated |
+| `se` | Standard error of ATT |
+| `t_stat` | T-statistic |
+| `p_value` | P-value for H0: ATT = 0 |
+| `conf_int` | Tuple of (lower, upper) confidence bounds |
+| `n_obs` | Total number of observations |
+| `n_treated_eligible` | Obs in treated group & eligible partition |
+| `n_treated_ineligible` | Obs in treated group & ineligible partition |
+| `n_control_eligible` | Obs in control group & eligible partition |
+| `n_control_ineligible` | Obs in control group & ineligible partition |
+| `estimation_method` | Method used ('dr', 'reg', or 'ipw') |
+| `group_means` | Dict of cell means for diagnostics |
+| `pscore_stats` | Propensity score statistics (IPW/DR only) |
+| `is_significant` | Boolean for significance at alpha |
+| `significance_stars` | String of significance stars |
+
+**Methods:**
+
+| Method | Description |
+|--------|-------------|
+| `summary(alpha)` | Get formatted summary string |
+| `print_summary(alpha)` | Print summary to stdout |
+| `to_dict()` | Convert to dictionary |
+| `to_dataframe()` | Convert to pandas DataFrame |
+
 ### HonestDiD
 
 ```python
@@ -2024,6 +2152,18 @@ This library implements methods from the following scholarly works:
 
 - **Arkhangelsky, D., Athey, S., Hirshberg, D. A., Imbens, G. W., & Wager, S. (2021).** "Synthetic Difference-in-Differences." *American Economic Review*, 111(12), 4088-4118. [https://doi.org/10.1257/aer.20190159](https://doi.org/10.1257/aer.20190159)
 
+### Triple Difference (DDD)
+
+- **Ortiz-Villavicencio, M., & Sant'Anna, P. H. C. (2025).** "Better Understanding Triple Differences Estimators." *Working Paper*. [https://arxiv.org/abs/2505.09942](https://arxiv.org/abs/2505.09942)
+
+  This paper shows that common DDD implementations (taking the difference between two DiDs, or applying three-way fixed effects regressions) are generally invalid when identification requires conditioning on covariates. The `TripleDifference` class implements their regression adjustment, inverse probability weighting, and doubly robust estimators.
+
+- **Gruber, J. (1994).** "The Incidence of Mandated Maternity Benefits." *American Economic Review*, 84(3), 622-641. [https://www.jstor.org/stable/2118071](https://www.jstor.org/stable/2118071)
+
+  Classic paper introducing the Triple Difference design for policy evaluation.
+
+- **Olden, A., & Møen, J. (2022).** "The Triple Difference Estimator." *The Econometrics Journal*, 25(3), 531-553. [https://doi.org/10.1093/ectj/utac010](https://doi.org/10.1093/ectj/utac010)
+
 ### Parallel Trends and Pre-Trend Testing
 
 - **Roth, J. (2022).** "Pretest with Caution: Event-Study Estimates after Testing for Parallel Trends." *American Economic Review: Insights*, 4(3), 305-322. [https://doi.org/10.1257/aeri.20210236](https://doi.org/10.1257/aeri.20210236)

ROADMAP.md

@@ -6,19 +6,19 @@ For past changes and release history, see [CHANGELOG.md](CHANGELOG.md).
 
 ---
 
-## Current Status (v1.2.0)
+## Current Status (v1.3.0)
 
 diff-diff is a **production-ready** DiD library with feature parity with R's `did` + `HonestDiD` ecosystem for core DiD analysis:
 
-- **Core estimators**: Basic DiD, TWFE, MultiPeriod, Callaway-Sant'Anna, Sun-Abraham, Synthetic DiD
+- **Core estimators**: Basic DiD, TWFE, MultiPeriod, Callaway-Sant'Anna, Sun-Abraham, Synthetic DiD, Triple Difference (DDD)
 - **Valid inference**: Robust SEs, cluster SEs, wild bootstrap, multiplier bootstrap
 - **Assumption diagnostics**: Parallel trends tests, placebo tests, Goodman-Bacon decomposition
 - **Sensitivity analysis**: Honest DiD (Rambachan-Roth), Pre-trends power analysis (Roth 2022)
 - **Study design**: Power analysis tools
 
 ---
 
-## Near-Term Enhancements (v1.3)
+## Near-Term Enhancements (v1.4)
 
 High-value additions building on our existing foundation.
 
@@ -42,14 +42,14 @@ Two-stage approach gaining traction in applied work. First residualizes outcomes
 
 **Reference**: Gardner (2022). *Working Paper*.
 
-### Triple Difference (DDD) Estimators
+### Staggered Triple Difference (DDD)
 
-Extends DiD to settings requiring a third differencing dimension. Common DDD implementations are invalid when covariates are needed for identification.
+Extend the existing `TripleDifference` estimator to handle staggered adoption settings.
 
-- Regression adjustment, IPW, and doubly robust DDD estimators
-- Staggered adoption support with multiple comparison groups
-- Proper covariate integration (naive "two DiD difference" approaches fail)
-- Bias reduction and precision gains over standard approaches
+- Group-time ATT(g,t) for DDD designs with variation in treatment timing
+- Event study aggregation for dynamic treatment effects
+- Multiple comparison groups (never-treated, not-yet-treated in either dimension)
+- Bootstrap inference for staggered DDD
 
 **Reference**: [Ortiz-Villavicencio & Sant'Anna (2025)](https://arxiv.org/abs/2505.09942). *Working Paper*. R package: `triplediff`.
 
@@ -61,7 +61,7 @@ Extends DiD to settings requiring a third differencing dimension. Common DDD imp
 
 ---
 
-## Medium-Term Enhancements (v1.4+)
+## Medium-Term Enhancements (v1.5+)
 
 Extending diff-diff to handle more complex settings.
 

diff_diff/__init__.py

@@ -81,6 +81,11 @@
     SunAbraham,
     SunAbrahamResults,
 )
+from diff_diff.triple_diff import (
+    TripleDifference,
+    TripleDifferenceResults,
+    triple_difference,
+)
 from diff_diff.utils import (
     WildBootstrapResults,
     check_parallel_trends,
@@ -107,6 +112,7 @@
     "SyntheticDiD",
     "CallawaySantAnna",
     "SunAbraham",
+    "TripleDifference",
     # Bacon Decomposition
     "BaconDecomposition",
     "BaconDecompositionResults",
@@ -123,6 +129,8 @@
     "GroupTimeEffect",
     "SunAbrahamResults",
     "SABootstrapResults",
+    "TripleDifferenceResults",
+    "triple_difference",
     # Visualization
     "plot_event_study",
     "plot_group_effects",