# Copyright 2022 - 2026 The PyMC Labs Developers
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Pretest/posttest nonequivalent group design."""
import warnings
from typing import Any, Literal
import arviz as az
import numpy as np
import pandas as pd
import seaborn as sns
import xarray as xr
from matplotlib import pyplot as plt
from patsy import build_design_matrices
from causalpy.constants import HDI_PROB, LEGEND_FONT_SIZE
from causalpy.custom_exceptions import (
DataException,
)
from causalpy.experiments.model_adapter import build_coords
from causalpy.formula_utils import build_formula_matrices
from causalpy.plot_utils import _PosteriorPlotStyle, plot_posterior_over_x
from causalpy.pymc_models import LinearRegression, PyMCModel
from causalpy.reporting import EffectSummary, _effect_summary_did
from causalpy.utils import _is_variable_dummy_coded, round_num
from .base import BaseExperiment
[docs]
class PrePostNEGD(BaseExperiment):
"""
A class to analyse data from pretest/posttest designs.
Parameters
----------
data : pd.DataFrame
A pandas dataframe.
formula : str
A statistical model formula.
group_variable_name : str
Name of the column in ``data`` for the group variable; should be
either binary or boolean.
pretreatment_variable_name : str
Name of the column in ``data`` for the pretreatment variable.
model : PyMCModel, optional
A PyMC model. Defaults to :class:`LinearRegression`.
**kwargs
Additional keyword arguments forwarded to :class:`BaseExperiment`.
Examples
--------
>>> import causalpy as cp
>>> df = cp.load_data("anova1")
>>> seed = 42
>>> result = cp.PrePostNEGD(
... df,
... formula="post ~ 1 + C(group) + pre",
... group_variable_name="group",
... pretreatment_variable_name="pre",
... model=cp.pymc_models.LinearRegression(
... sample_kwargs={
... "target_accept": 0.95,
... "random_seed": seed,
... "progressbar": False,
... }
... ),
... )
>>> result.summary(round_to=1) # doctest: +SKIP
==================Pretest/posttest Nonequivalent Group Design===================
Formula: post ~ 1 + C(group) + pre
<BLANKLINE>
Results:
Causal impact = 2, $CI_{94%}$[2, 2]
Model coefficients:
Intercept -0.5, 94% HDI [-1, 0.2]
C(group)[T.1] 2, 94% HDI [2, 2]
pre 1, 94% HDI [1, 1]
y_hat_sigma 0.5, 94% HDI [0.5, 0.6]
"""
supports_ols = False
supports_bayes = True
_default_model_class = LinearRegression
_deprecated_design_aliases = {"X": ("design", "X"), "y": ("design", "y")}
[docs]
def __init__(
self,
data: pd.DataFrame,
formula: str,
group_variable_name: str,
pretreatment_variable_name: str,
model: PyMCModel | None = None,
**kwargs: Any,
) -> None:
super().__init__(model=model)
self.causal_impact: xr.DataArray
self.pred_xi: np.ndarray
self.pred_untreated: az.InferenceData
self.pred_treated: az.InferenceData
self.data = data
self.expt_type = "Pretest/posttest Nonequivalent Group Design"
self.formula = formula
self.group_variable_name = group_variable_name
self.pretreatment_variable_name = pretreatment_variable_name
self.input_validation()
self._build_design_matrices()
self._prepare_data()
self.algorithm()
def _build_design_matrices(self) -> None:
"""Build design matrices from formula and data using patsy."""
y, X = build_formula_matrices(self.formula, self.data)
self._y_design_info = y.design_info
self._x_design_info = X.design_info
self.labels = X.design_info.column_names
self._y_raw, self._X_raw = np.asarray(y), np.asarray(X)
self.outcome_variable_name = y.design_info.column_names[0]
def _prepare_data(self) -> None:
"""Bundle design matrices into an ``xr.Dataset``."""
self.design = self._build_design_dataset(
self._X_raw,
self._y_raw,
obs_ind=self.data.index,
coeffs=self.labels,
)
del self._X_raw, self._y_raw
[docs]
def algorithm(self) -> None:
"""Run the experiment algorithm: fit model, predict, and calculate causal impact."""
X = self.design["X"]
y = self.design["y"]
if self._model_backend.is_ols:
raise NotImplementedError("Not implemented for OLS model")
if not self._model_backend.is_bayesian:
raise ValueError("Model type not recognized")
self._model_backend.fit(
X=X,
y=y,
coords=build_coords(self.labels, X.shape[0]),
)
assert self.model.idata is not None
# Calculate the posterior predictive for the treatment and control for an
# interpolated set of pretest values
# get the model predictions of the observed data
self.pred_xi = np.linspace(
np.min(self.data[self.pretreatment_variable_name]),
np.max(self.data[self.pretreatment_variable_name]),
200,
)
# untreated
x_pred_untreated = pd.DataFrame(
{
self.pretreatment_variable_name: self.pred_xi,
self.group_variable_name: np.zeros(self.pred_xi.shape),
}
)
(new_x_untreated,) = build_design_matrices(
[self._x_design_info], x_pred_untreated
)
self.pred_untreated = self.model.predict(X=np.asarray(new_x_untreated))
# treated
x_pred_treated = pd.DataFrame(
{
self.pretreatment_variable_name: self.pred_xi,
self.group_variable_name: np.ones(self.pred_xi.shape),
}
)
(new_x_treated,) = build_design_matrices([self._x_design_info], x_pred_treated)
self.pred_treated = self.model.predict(X=np.asarray(new_x_treated))
# Evaluate causal impact as equal to the treatment effect
self.causal_impact = self.model.idata.posterior["beta"].sel(
{"coeffs": self._get_treatment_effect_coeff()}
)
[docs]
def input_validation(self) -> None:
"""Validate the input data and model formula for correctness."""
if not _is_variable_dummy_coded(self.data[self.group_variable_name]):
raise DataException(
f"""
There must be 2 levels of the grouping variable
{self.group_variable_name}. I.e. the treated and untreated.
"""
)
def _get_treatment_effect_coeff(self) -> str:
"""Find the beta regression coefficient corresponding to the
group (i.e. treatment) effect.
For example if self.group_variable_name is 'group' and
the labels are `['Intercept', 'C(group)[T.1]', 'pre']`
then we want `C(group)[T.1]`.
"""
for label in self.labels:
if (self.group_variable_name in label) & (":" not in label):
return label
raise NameError("Unable to find coefficient name for the treatment effect")
def _causal_impact_summary_stat(self, round_to: int | None = 2) -> str:
"""Computes the mean and credible interval bounds for the causal impact."""
percentiles = self.causal_impact.quantile(
[(1 - HDI_PROB) / 2, 1 - (1 - HDI_PROB) / 2]
).values
ci = (
rf"$CI_{{{HDI_PROB * 100:.0f}\%}}$"
+ f"[{round_num(percentiles[0], round_to)}, {round_num(percentiles[1], round_to)}]"
)
causal_impact = f"{round_num(self.causal_impact.mean(), round_to)}, "
return f"Causal impact = {causal_impact + ci}"
[docs]
def summary(self, round_to: int | None = None) -> None:
"""Print summary of main results and model coefficients.
Parameters
----------
round_to : int, optional
Number of decimals used to round results. Defaults to 2. Use
``None`` to return raw numbers.
"""
print(f"{self.expt_type:=^80}")
print(f"Formula: {self.formula}")
print("\nResults:")
print(self._causal_impact_summary_stat(round_to))
self.print_coefficients(round_to)
[docs]
def plot(
self,
*,
round_to: int | None = None,
ci_prob: float = HDI_PROB,
hdi_prob: float | None = None,
kind: Literal["ribbon", "histogram", "spaghetti"] = "ribbon",
ci_kind: Literal["hdi", "eti"] = "hdi",
num_samples: int = 50,
figsize: tuple[float, float] = (7, 9),
show: bool = True,
legend_kwargs: dict[str, Any] | None = None,
) -> tuple[plt.Figure, list[plt.Axes]]:
"""Plot the pre-post non-equivalent group design results.
Parameters
----------
round_to : int, optional
Number of decimals used to round numerical results in the figure.
Defaults to ``None``, in which case 2 significant figures are
used.
ci_prob : float
Probability mass of the highest density interval drawn around the
posterior predictive bands for the control and treatment groups,
and around the posterior of the estimated treatment effect.
Must be in ``(0, 1]``. Defaults to
:data:`~causalpy.constants.HDI_PROB` (currently 0.94).
hdi_prob : float, optional
Deprecated. Use ``ci_prob`` instead.
kind : {"ribbon", "histogram", "spaghetti"}, optional
How posterior uncertainty is rendered via
:func:`~causalpy.plot_utils.plot_posterior_over_x`. Defaults to ``"ribbon"``.
For ``"spaghetti"``, legends use draw lines rather than a shaded
band. For ``"histogram"``, uncertainty is shown as a 2D density
heatmap with a mean line overlay (no ribbon patch for legends).
ci_kind : {"hdi", "eti"}, optional
Credible interval type when ``kind="ribbon"``. Defaults to
``"hdi"``.
num_samples : int, optional
Number of posterior draws when ``kind="spaghetti"``. Defaults
to 50. Ignored for other kinds.
figsize : tuple of (float, float)
Width and height of the figure in inches, passed to
:func:`matplotlib.pyplot.subplots`. Defaults to ``(7, 9)``.
show : bool
Whether to automatically display the plot. Defaults to ``True``.
legend_kwargs : dict, optional
Keyword arguments to adjust legend placement and styling.
Supported keys: ``loc``, ``bbox_to_anchor``, ``fontsize``,
``frameon``, ``title`` (``bbox_transform`` is accepted alongside
``bbox_to_anchor``). The existing legend is modified **in
place** so that custom handles are preserved.
Returns
-------
fig : matplotlib.figure.Figure
The figure that was created.
ax : list[matplotlib.axes.Axes]
The two axes (top: scatter and posterior predictive bands,
bottom: estimated treatment effect posterior).
"""
if hdi_prob is not None:
warnings.warn(
"hdi_prob is deprecated and will be removed in a future release. "
"Use ci_prob instead.",
FutureWarning,
stacklevel=2,
)
ci_prob = hdi_prob
return self._render_plot(
show=show,
legend_kwargs=legend_kwargs,
round_to=round_to,
ci_prob=ci_prob,
kind=kind,
ci_kind=ci_kind,
num_samples=num_samples,
figsize=figsize,
)
def _bayesian_plot(
self,
round_to: int | None = None,
ci_prob: float = HDI_PROB,
kind: Literal["ribbon", "histogram", "spaghetti"] = "ribbon",
ci_kind: Literal["hdi", "eti"] = "hdi",
num_samples: int = 50,
figsize: tuple[float, float] = (7, 9),
**kwargs: Any,
) -> tuple[plt.Figure, list[plt.Axes]]:
"""Generate plot for ANOVA-like experiments with non-equivalent group designs.
Parameters
----------
round_to : int, optional
Number of decimals used to round results. Defaults to 2. Use ``None``
to return raw numbers.
hdi_prob : float, optional
Probability mass of the highest density interval drawn around the
posterior predictive bands for the control and treatment groups,
and around the posterior of the estimated treatment effect.
Must be in ``(0, 1]``. Defaults to
:data:`~causalpy.constants.HDI_PROB` (currently 0.94).
figsize : tuple of (float, float), optional
Width and height of the figure in inches. Defaults to ``(7, 9)``.
"""
style: _PosteriorPlotStyle = {
"ci_prob": ci_prob,
"kind": kind,
"ci_kind": ci_kind,
"num_samples": num_samples,
}
fig, ax = plt.subplots(
2, 1, figsize=figsize, gridspec_kw={"height_ratios": [3, 1]}
)
# Plot raw data
sns.scatterplot(
x="pre",
y="post",
hue="group",
alpha=0.5,
data=self.data,
legend=True,
ax=ax[0],
)
ax[0].set(xlabel="Pretest", ylabel="Posttest")
# plot posterior predictive of untreated
h_line, h_patch = plot_posterior_over_x(
self.pred_xi,
self.pred_untreated["posterior_predictive"].mu.isel(treated_units=0),
ax=ax[0],
**style,
plot_hdi_kwargs={"color": "C0"},
label="Control group",
)
handles = [(h_line, h_patch)]
labels = ["Control group"]
# plot posterior predictive of treated
h_line, h_patch = plot_posterior_over_x(
self.pred_xi,
self.pred_treated["posterior_predictive"].mu.isel(treated_units=0),
ax=ax[0],
**style,
plot_hdi_kwargs={"color": "C1"},
label="Treatment group",
)
handles.append((h_line, h_patch))
labels.append("Treatment group")
ax[0].legend(
handles=(h_tuple for h_tuple in handles),
labels=labels,
fontsize=LEGEND_FONT_SIZE,
)
# Plot estimated caual impact / treatment effect
az.plot_posterior(
self.causal_impact,
ref_val=0,
ax=ax[1],
round_to=round_to,
hdi_prob=ci_prob,
)
ax[1].set(title="Estimated treatment effect")
return fig, ax
[docs]
def effect_summary(
self,
*,
direction: Literal["increase", "decrease", "two-sided"] = "increase",
alpha: float = 0.05,
min_effect: float | None = None,
**kwargs: Any,
) -> EffectSummary:
"""
Generate a decision-ready summary of causal effects for PrePostNEGD.
Parameters
----------
direction : {"increase", "decrease", "two-sided"}, default="increase"
Direction for tail probability calculation (PyMC only).
alpha : float, default=0.05
Significance level for HDI/CI intervals (1-alpha confidence level).
min_effect : float, optional
Region of Practical Equivalence (ROPE) threshold (PyMC only).
**kwargs
Reserved for forward-compatibility; not consumed by this
implementation.
Returns
-------
EffectSummary
Object with .table (DataFrame) and .text (str) attributes
"""
return _effect_summary_did(
self,
direction=direction,
alpha=alpha,
min_effect=min_effect,
)
