Add ContinuousDiD estimator (Callaway, Goodman-Bacon & Sant'Anna 2024)

TODO.md

@@ -44,6 +44,7 @@ Deferred items from PR reviews that were not addressed before merge.
 | Issue | Location | PR | Priority |
 |-------|----------|----|----------|
 | ImputationDiD dense `(A0'A0).toarray()` scales O((U+T+K)^2), OOM risk on large panels | `imputation.py` | #141 | Medium (deferred — only triggers when sparse solver fails; fixing requires sparse least-squares alternatives) |
+| Bootstrap NaN-gating gap: manual SE/CI/p-value without non-finite filtering or SE<=0 guard | `imputation_bootstrap.py`, `two_stage_bootstrap.py` | #177 | Medium — migrate to `compute_effect_bootstrap_stats` from `bootstrap_utils.py` |
 
 #### Performance
 

diff_diff/__init__.py

@@ -70,6 +70,7 @@
     aggregate_to_cohorts,
     balance_panel,
     create_event_time,
+    generate_continuous_did_data,
     generate_did_data,
     generate_ddd_data,
     generate_event_study_data,
@@ -122,6 +123,11 @@
     TripleDifferenceResults,
     triple_difference,
 )
+from diff_diff.continuous_did import (
+    ContinuousDiD,
+    ContinuousDiDResults,
+    DoseResponseCurve,
+)
 from diff_diff.trop import (
     TROP,
     TROPResults,
@@ -161,6 +167,7 @@
     "MultiPeriodDiD",
     "SyntheticDiD",
     "CallawaySantAnna",
+    "ContinuousDiD",
     "SunAbraham",
     "ImputationDiD",
     "TwoStageDiD",
@@ -181,6 +188,8 @@
     "CallawaySantAnnaResults",
     "CSBootstrapResults",
     "GroupTimeEffect",
+    "ContinuousDiDResults",
+    "DoseResponseCurve",
     "SunAbrahamResults",
     "SABootstrapResults",
     "ImputationDiDResults",
@@ -228,6 +237,7 @@
     "generate_ddd_data",
     "generate_panel_data",
     "generate_event_study_data",
+    "generate_continuous_did_data",
     "create_event_time",
     "aggregate_to_cohorts",
     "rank_control_units",

diff_diff/bootstrap_utils.py

@@ -0,0 +1,279 @@
+"""
+Shared bootstrap utilities for multiplier bootstrap inference.
+
+Provides weight generation, percentile CI, and p-value helpers used by
+both CallawaySantAnna and ContinuousDiD estimators.
+"""
+
+import warnings
+from typing import Optional, Tuple
+
+import numpy as np
+
+from diff_diff._backend import HAS_RUST_BACKEND, _rust_bootstrap_weights
+
+__all__ = [
+    "generate_bootstrap_weights",
+    "generate_bootstrap_weights_batch",
+    "generate_bootstrap_weights_batch_numpy",
+    "compute_percentile_ci",
+    "compute_bootstrap_pvalue",
+    "compute_effect_bootstrap_stats",
+]
+
+
+def generate_bootstrap_weights(
+    n_units: int,
+    weight_type: str,
+    rng: np.random.Generator,
+) -> np.ndarray:
+    """
+    Generate bootstrap weights for multiplier bootstrap.
+
+    Parameters
+    ----------
+    n_units : int
+        Number of units (clusters) to generate weights for.
+    weight_type : str
+        Type of weights: "rademacher", "mammen", or "webb".
+    rng : np.random.Generator
+        Random number generator.
+
+    Returns
+    -------
+    np.ndarray
+        Array of bootstrap weights with shape (n_units,).
+    """
+    if weight_type == "rademacher":
+        return rng.choice([-1.0, 1.0], size=n_units)
+    elif weight_type == "mammen":
+        sqrt5 = np.sqrt(5)
+        val1 = -(sqrt5 - 1) / 2
+        val2 = (sqrt5 + 1) / 2
+        p1 = (sqrt5 + 1) / (2 * sqrt5)
+        return rng.choice([val1, val2], size=n_units, p=[p1, 1 - p1])
+    elif weight_type == "webb":
+        values = np.array([
+            -np.sqrt(3 / 2), -np.sqrt(2 / 2), -np.sqrt(1 / 2),
+            np.sqrt(1 / 2), np.sqrt(2 / 2), np.sqrt(3 / 2)
+        ])
+        return rng.choice(values, size=n_units)
+    else:
+        raise ValueError(
+            f"weight_type must be 'rademacher', 'mammen', or 'webb', "
+            f"got '{weight_type}'"
+        )
+
+
+def generate_bootstrap_weights_batch(
+    n_bootstrap: int,
+    n_units: int,
+    weight_type: str,
+    rng: np.random.Generator,
+) -> np.ndarray:
+    """
+    Generate all bootstrap weights at once (vectorized).
+
+    Uses Rust backend if available for parallel generation.
+
+    Parameters
+    ----------
+    n_bootstrap : int
+        Number of bootstrap iterations.
+    n_units : int
+        Number of units (clusters) to generate weights for.
+    weight_type : str
+        Type of weights: "rademacher", "mammen", or "webb".
+    rng : np.random.Generator
+        Random number generator.
+
+    Returns
+    -------
+    np.ndarray
+        Array of bootstrap weights with shape (n_bootstrap, n_units).
+    """
+    if HAS_RUST_BACKEND and _rust_bootstrap_weights is not None:
+        seed = rng.integers(0, 2**63 - 1)
+        return _rust_bootstrap_weights(n_bootstrap, n_units, weight_type, seed)
+    return generate_bootstrap_weights_batch_numpy(n_bootstrap, n_units, weight_type, rng)
+
+
+def generate_bootstrap_weights_batch_numpy(
+    n_bootstrap: int,
+    n_units: int,
+    weight_type: str,
+    rng: np.random.Generator,
+) -> np.ndarray:
+    """
+    NumPy fallback implementation of :func:`generate_bootstrap_weights_batch`.
+
+    Parameters
+    ----------
+    n_bootstrap : int
+        Number of bootstrap iterations.
+    n_units : int
+        Number of units (clusters) to generate weights for.
+    weight_type : str
+        Type of weights: "rademacher", "mammen", or "webb".
+    rng : np.random.Generator
+        Random number generator.
+
+    Returns
+    -------
+    np.ndarray
+        Array of bootstrap weights with shape (n_bootstrap, n_units).
+    """
+    if weight_type == "rademacher":
+        return rng.choice([-1.0, 1.0], size=(n_bootstrap, n_units))
+    elif weight_type == "mammen":
+        sqrt5 = np.sqrt(5)
+        val1 = -(sqrt5 - 1) / 2
+        val2 = (sqrt5 + 1) / 2
+        p1 = (sqrt5 + 1) / (2 * sqrt5)
+        return rng.choice([val1, val2], size=(n_bootstrap, n_units), p=[p1, 1 - p1])
+    elif weight_type == "webb":
+        values = np.array([
+            -np.sqrt(3 / 2), -np.sqrt(2 / 2), -np.sqrt(1 / 2),
+            np.sqrt(1 / 2), np.sqrt(2 / 2), np.sqrt(3 / 2)
+        ])
+        return rng.choice(values, size=(n_bootstrap, n_units))
+    else:
+        raise ValueError(
+            f"weight_type must be 'rademacher', 'mammen', or 'webb', "
+            f"got '{weight_type}'"
+        )
+
+
+def compute_percentile_ci(
+    boot_dist: np.ndarray,
+    alpha: float,
+) -> Tuple[float, float]:
+    """
+    Compute percentile confidence interval from bootstrap distribution.
+
+    Parameters
+    ----------
+    boot_dist : np.ndarray
+        Bootstrap distribution (1-D array).
+    alpha : float
+        Significance level (e.g., 0.05 for 95% CI).
+
+    Returns
+    -------
+    tuple of float
+        ``(lower, upper)`` confidence interval bounds.
+    """
+    lower = float(np.percentile(boot_dist, alpha / 2 * 100))
+    upper = float(np.percentile(boot_dist, (1 - alpha / 2) * 100))
+    return (lower, upper)
+
+
+def compute_bootstrap_pvalue(
+    original_effect: float,
+    boot_dist: np.ndarray,
+    n_valid: Optional[int] = None,
+) -> float:
+    """
+    Compute two-sided bootstrap p-value using the percentile method.
+
+    Parameters
+    ----------
+    original_effect : float
+        Original point estimate.
+    boot_dist : np.ndarray
+        Bootstrap distribution of the effect.
+    n_valid : int, optional
+        Number of valid bootstrap samples for p-value floor.
+        If None, uses ``len(boot_dist)``.
+
+    Returns
+    -------
+    float
+        Two-sided bootstrap p-value.
+    """
+    if original_effect >= 0:
+        p_one_sided = np.mean(boot_dist <= 0)
+    else:
+        p_one_sided = np.mean(boot_dist >= 0)
+
+    p_value = min(2 * p_one_sided, 1.0)
+    n_for_floor = n_valid if n_valid is not None else len(boot_dist)
+    p_value = max(p_value, 1 / (n_for_floor + 1))
+    return float(p_value)
+
+
+def compute_effect_bootstrap_stats(
+    original_effect: float,
+    boot_dist: np.ndarray,
+    alpha: float = 0.05,
+    context: str = "bootstrap distribution",
+) -> Tuple[float, Tuple[float, float], float]:
+    """
+    Compute bootstrap statistics for a single effect.
+
+    Filters non-finite samples, returning NaN for all statistics if
+    fewer than 50% of samples are valid.
+
+    Parameters
+    ----------
+    original_effect : float
+        Original point estimate.
+    boot_dist : np.ndarray
+        Bootstrap distribution of the effect.
+    alpha : float, default=0.05
+        Significance level.
+    context : str, optional
+        Description for warning messages.
+
+    Returns
+    -------
+    se : float
+        Bootstrap standard error.
+    ci : tuple of float
+        Percentile confidence interval.
+    p_value : float
+        Bootstrap p-value.
+    """
+    if not np.isfinite(original_effect):
+        return np.nan, (np.nan, np.nan), np.nan
+
+    finite_mask = np.isfinite(boot_dist)
+    n_valid = np.sum(finite_mask)
+    n_total = len(boot_dist)
+
+    if n_valid < n_total:
+        n_nonfinite = n_total - n_valid
+        warnings.warn(
+            f"Dropping {n_nonfinite}/{n_total} non-finite bootstrap samples "
+            f"in {context}. Bootstrap estimates based on remaining valid samples.",
+            RuntimeWarning,
+            stacklevel=3,
+        )
+
+    if n_valid < n_total * 0.5:
+        warnings.warn(
+            f"Too few valid bootstrap samples ({n_valid}/{n_total}) in {context}. "
+            "Returning NaN for SE/CI/p-value to signal invalid inference.",
+            RuntimeWarning,
+            stacklevel=3,
+        )
+        return np.nan, (np.nan, np.nan), np.nan
+
+    valid_dist = boot_dist[finite_mask]
+    se = float(np.std(valid_dist, ddof=1))
+
+    # Guard: if SE is not finite or zero, all inference fields must be NaN.
+    if not np.isfinite(se) or se <= 0:
+        warnings.warn(
+            f"Bootstrap SE is non-finite or zero (n_valid={n_valid}) in {context}. "
+            "Returning NaN for SE/CI/p-value.",
+            RuntimeWarning,
+            stacklevel=3,
+        )
+        return np.nan, (np.nan, np.nan), np.nan
+
+    ci = compute_percentile_ci(valid_dist, alpha)
+    p_value = compute_bootstrap_pvalue(
+        original_effect, valid_dist, n_valid=len(valid_dist)
+    )
+    return se, ci, p_value