Address PR #208 review: cap SyntheticDiD treatment_fraction to fix zero-power bug

Add _sdid_dgp_kwargs that caps treatment_fraction at 0.4 so the placebo
variance method has enough pseudo-controls (n_control > n_treated).
Add regression test for the default SyntheticDiD path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/power.py

@@ -109,6 +109,27 @@ def _factor_dgp_kwargs(
     )
 
 
+def _sdid_dgp_kwargs(
+    n_units: int,
+    n_periods: int,
+    treatment_effect: float,
+    treatment_fraction: float,
+    treatment_period: int,
+    sigma: float,
+) -> Dict[str, Any]:
+    # SyntheticDiD placebo variance requires n_control > n_treated;
+    # cap at 40% treated to ensure adequate pseudo-controls.
+    safe_fraction = min(treatment_fraction, 0.4)
+    return _factor_dgp_kwargs(
+        n_units=n_units,
+        n_periods=n_periods,
+        treatment_effect=treatment_effect,
+        treatment_fraction=safe_fraction,
+        treatment_period=treatment_period,
+        sigma=sigma,
+    )
+
+
 def _ddd_dgp_kwargs(
     n_units: int,
     n_periods: int,
@@ -315,7 +336,7 @@ def _get_registry() -> Dict[str, _EstimatorProfile]:
         ),
         "SyntheticDiD": _EstimatorProfile(
             default_dgp=generate_factor_data,
-            dgp_kwargs_builder=_factor_dgp_kwargs,
+            dgp_kwargs_builder=_sdid_dgp_kwargs,
             fit_kwargs_builder=_sdid_fit_kwargs,
             result_extractor=_extract_simple,
             min_n=30,

tests/test_power.py

@@ -40,6 +40,7 @@
     _extract_staggered,
     _factor_dgp_kwargs,
     _get_registry,
+    _sdid_dgp_kwargs,
     _staggered_dgp_kwargs,
     _staggered_fit_kwargs,
     _trop_fit_kwargs,
@@ -707,6 +708,7 @@ def test_dgp_kwargs_builders_return_dicts(self):
             _basic_dgp_kwargs,
             _staggered_dgp_kwargs,
             _factor_dgp_kwargs,
+            _sdid_dgp_kwargs,
             _ddd_dgp_kwargs,
         ]:
             result = builder(**params)
@@ -969,6 +971,21 @@ def test_synthetic_did(self):
         )
         self._assert_valid_result(result, "SyntheticDiD")
 
+    @pytest.mark.slow
+    def test_synthetic_did_default_fraction(self):
+        """Default treatment_fraction=0.5 must not produce zero power."""
+        result = simulate_power(
+            SyntheticDiD(),
+            n_units=50,
+            n_periods=6,
+            treatment_period=3,
+            n_simulations=10,
+            seed=42,
+            progress=False,
+        )
+        self._assert_valid_result(result, "SyntheticDiD")
+        assert result.power > 0, "Default SyntheticDiD path gave zero power"
+
 
 # ---------------------------------------------------------------------------
 # simulate_mde tests