Sigma

Sigma turns raw tabular data into a readable, statistically grounded binary tree. It is built to give a statistically robust, quick, fully automatic look at a dataset. It is not designed to compete with state-of-the-art but complex and time-consuming machine learning models on raw predictive performance. It is a good fit for reliable diagnostics on new datasets, one-off segmentation studies, and model drift investigations, and it helps examine the outputs of more complex models before relying on them.

scikit-learn's decision trees tend to be biased in their variable selection, require manual tuning of depth and minimum-samples thresholds to stay readable, and routinely grow into hairy, hard-to-interpret structures. Under the hood they pick splits by exhaustively optimizing a purity measure, which biases selection toward features with many possible split points and gives no statistically grounded stopping rule. Sigma uses permutation-based conditional-inference splits instead: at every node it tests each candidate variable for a real association with the response, picks the strongest, and stops the moment none is statistically significant. The tree finds its own depth automatically: no manual cutoff, no rule of thumb on samples per leaf. The single knob is the false-positive rate of each split test (the alpha level, 5% by default); raise it for a deeper exploratory tree, lower it for a stricter one. The result is a tree you can trust as a diagnostic.

Yes, but I have something better. Fitting a Sigma tree gives you a robust first-approximation predictive model with very little effort: no tuning, no feature engineering, no encoding of categoricals, and a tree you can read end to end. It is a sound baseline you can deploy and trust. The tradeoff is subtlety for simplicity. A Sigma tree captures the dominant structure in the data but misses the finer interactions and gradients a richer model could exploit. When you need that accuracy without giving up interpretability, reach for Tau, my state-of-the-art engine for tabular data. It turns weak signals into outstanding predictions.

Yes. ClassificationTree, RegressionTree, and SurvivalTree implement the standard scikit-learn estimator API, so they slot into Pipeline, ColumnTransformer, GridSearchCV, cross-validation utilities, and model serialization without any custom glue. Pandas DataFrames are first-class inputs: pass them directly to fit and predict, declare categorical columns by name, and keep your column metadata throughout training and inference. NumPy arrays are of course also supported. Categorical features are handled natively (no upstream one-hot encoding needed), and sample weights are fully supported through all the statistical significance tests.

Sigma supports the classic confidence intervals; the right one depends on the coverage guarantees you want. For classification: Clopper-Pearson, Jeffreys, and Wilson. For regression: Bayesian bootstrap, normal, Student-t, log-normal, gamma, Poisson, exponential, and beta. For survival outcomes: Brookmeyer-Crowley for the median survival time, log-log Greenwood for the survival probability at a fixed time, and Klein-Moeschberger integrated Greenwood for the restricted mean survival time (RMST). You pick the one that best fits your target's distribution.

On small datasets, the statistical stopping rule and the confidence intervals on leaves keep the tree honest about what it does and does not know. On larger datasets, fit time scales linearly with the number of observations and with the number of candidate variables, so Sigma's runtimes stay predictable as the data grows.

Python 3.10 and above, on every platform supported by NumPy and SciPy. Sigma depends only on NumPy, SciPy, and scikit-learn. Graphviz rendering is available as an optional extra for visualization, but it is not required to fit or use models.

A single pip install ars-sigma command installs the library. There are no compiled extensions to build, which makes installation simple. Everything else (visualization examples, deployment notes, source, and issue tracker) is on the GitHub repository.

Sigma is source-available: under the Sigma Source License you can freely read, use, modify, and redistribute the library, provided the license file and attribution travel with every copy. This is, however, not open source: ArsChitectura SAS retains all rights and may revoke the license at will. For broader rights and professional support, a commercial license is available on request through the contact page. Contributions, issues, and feedback are welcome on the GitHub repository.

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