Sujet : data
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Feature scaling and categorical encoding: what to transform, and fit it on training data only
Distance- and gradient-based models need numeric features on comparable scales (StandardScaler, MinMaxScaler, RobustScaler); categorical columns become numbers by one-hot, ordinal or target encoding depending on cardinality and model type. Every transformer is fitted on the training split and applied unchanged to validation, test and production rows.
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Training, validation and test sets: what each split is for and how to cut it
A model is fitted on the training set, choices between models and hyperparameters are made on the validation set (or by cross-validation), and the test set is touched once for the final estimate; how rows are assigned to splits (random, stratified, by group, by time) decides whether the estimate says anything about production.
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Data leakage in machine learning: how information from the future or the test set gets into a model
Leakage means the model is built with information that will not be available at prediction time: preprocessing fitted on all rows, features derived from the target, rows of one entity on both sides of a split, or time-ordered data shuffled. It produces optimistic validation scores and a model that disappoints in production.
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Provenance and versioning for small datasets
Keep raw data immutable with checksums and a recorded origin, derive new files with scripts instead of editing, version data pointers with the code (DVC or Git LFS), describe the package with a datapackage.json, and note provenance in PROV terms so that every figure in a report resolves to a commit, a checksum and a script.
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Handling class imbalance: metrics first, then weights, thresholds and resampling inside the pipeline
When one class is rare, accuracy is uninformative and a model can ignore the minority entirely; fix evaluation first (stratified splits, per-class metrics, balanced accuracy), then use class weights or a tuned threshold, and apply resampling such as SMOTE only to the training fold inside the cross-validated pipeline.
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Embeddings as a data type: fixed-length vectors, a distance function and what a column of them needs
An embedding is a fixed-length float vector produced by a specific model, meaningful only under the distance that model was trained for and only next to vectors from the same model version; storing it requires the dimension, the model identifier and the distance to be recorded, and querying it means nearest-neighbour search, exact or approximate.
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