Least-Squares Shapley Performance Attribution (LS-SPA)

Installation - Usage - Hello world - Example notebook - Optional arguments - Citing

Library companion to the paper Efficient Shapley Performance Attribution for Least-Squares Regression by Logan Bell, Nikhil Devanathan, and Stephen Boyd.

The results provided in the reference paper were generated using a more performant, but harder to use implementation of the same algorithm. This benchmark code and the numerical experiments from the reference paper can be found at cvxgrp/ls-spa-benchmark. We recommend caution in trying to use the benchmark code.

Installation

To install this package, execute

bash pip install ls_spa

Import ls_spa by adding

python from ls_spa import ls_spa

to the top of your Python file.

ls_spa has the following dependencies:

• numpy
• scipy
• pandas

Optional dependencies are

• marimo for using the demo notebook
• matplotlib for plotting in the demo notebook

Usage

We assume that you have imported ls_spa and you have a $N\times p$ matrix of training data X_train, a $M\times p$ matrix of testing data X_test, a $N$ vector of training labels y_train, and a $M$ vector of testing labels y_test for positive integers $p, N, M$ with $N,M\geq p$. In this case, you can find the Shapley attribution of the out-of-sample $R^2$ on your data by executing

python attrs = ls_spa(X_train, X_test, y_train, y_test).attribution

attrs will be a JAX vector containing the Shapley values of your features. The ls_spa function computes Shapley values for the given data using the LS-SPA method described in the companion paper. It takes arguments:

• X_train: Training feature matrix.
• X_test: Testing feature matrix.
• y_train: Training response vector.
• y_test: Testing response vector.

Hello world

We present a complete Python script that utilizes LS-SPA to compute the Shapley attribution on the data from the toy example described in the companion paper.

```python

Imports

import numpy as np from lsspa import lsspa

Data loading

Xtrain, Xtest, ytrain, ytest = [np.load("./data/toydata.npz")[key] for key in ["Xtrain","Xtest","ytrain","y_test"]]

Compute Shapley attribution with LS-SPA

results = lsspa(Xtrain, Xtest, ytrain, y_test)

Print attribution

print(results) ```

This example uses data from the data directory of this repository.

The line print(results) prints a dashboard of information generated while computing the Shapley attribution such as the attribution, the $R^2$ of the model fitted with all of the features, the feature cofficients of the fitted model, and an error estimate on the attribution (since LS-SPA is a method of estimation).

To extract just the vector of Shapley values, use results.attribution. For more info, see optional arguments.

Example notebook

In this demo, we walk through the process of computing Shapley values on the data for the toy example in the companion paper. We then use ls_spa to compute the Shapley attribution on the same data.

Optional arguments

ls_spa takes the optional arguments:

• reg: Regularization parameter (Default 0).
• method: Permutation sampling method. Options include 'random', 'permutohedron', 'argsort', and 'exact'. If None, 'argsort' is used if the number of features is greater than 10; otherwise, 'exact' is used.
• batch_size: Number of permutations in each batch (Default 2**7).
• num_batches: Maximum number of batches (Default 2**7).
• tolerance: Convergence tolerance for the Shapley values (Default 1e-2).
• seed: Seed for random number generation (Default 42).
• return_history: Flag to determine whether to return the history of error estimates and attributions for each feature chain (Default False).

ls_spa returns a ShapleyResults object. The ShapleyResults object has the fields:

• attribution: Array of Shapley values for each feature.
• attribution_history: Array of Shapley values for each iteration. None if return_history=False in ls_spa call.
• theta: Array of regression coefficients.
• overall_error: Mean absolute error of the Shapley values.
• error_history: Array of mean absolute errors for each iteration. None if return_history=False in ls_spa call.
• attribution_errors: Array of absolute errors for each feature.
• r_squared: Out-of-sample R-squared statistic of the regression.

Citing

If you use this code for research, please cite the associated paper.

bibtex @article{Bell2024, title = {Efficient Shapley performance attribution for least-squares regression}, volume = {34}, ISSN = {1573-1375}, url = {http://dx.doi.org/10.1007/s11222-024-10459-9}, DOI = {10.1007/s11222-024-10459-9}, number = {5}, journal = {Statistics and Computing}, publisher = {Springer Science and Business Media LLC}, author = {Bell, Logan and Devanathan, Nikhil and Boyd, Stephen}, year = {2024}, month = jul }