Version Compatibility

Reference examples tested with: numpy 1.26+, scikit-learn 1.4+

Before using code patterns, verify installed versions match. If versions differ:

• Python:

  pip show <package>

  then

  help(module.function)

  to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Cross-Validation for Biomedical Data

"Properly validate my omics classifier" → Use nested cross-validation with stratified splits to get unbiased performance estimates while tuning hyperparameters on small biomedical datasets.

• Python:

  sklearn.model_selection.cross_val_score()

  with

  StratifiedKFold

  inner/outer loops

Why Nested CV Matters

Simple train/test splits overestimate performance on small omics datasets. Nested CV provides unbiased estimates by separating hyperparameter tuning from performance evaluation.

Nested Cross-Validation

Goal: Obtain unbiased performance estimates by separating hyperparameter tuning from evaluation.

Approach: Use an outer CV loop for scoring and an inner CV loop for grid search, preventing information leakage between tuning and evaluation.

from sklearn.model_selection import cross_val_score, StratifiedKFold, GridSearchCV
from sklearn.ensemble import RandomForestClassifier
from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import Pipeline
import numpy as np

pipe = Pipeline([
    ('scaler', StandardScaler()),
    ('clf', RandomForestClassifier(random_state=42))
])

param_grid = {
    'clf__n_estimators': [50, 100, 200],
    'clf__max_depth': [5, 10, None]
}

# Outer CV: performance estimation (5 folds)
# Inner CV: hyperparameter tuning (3 folds)
outer_cv = StratifiedKFold(n_splits=5, shuffle=True, random_state=42)
inner_cv = StratifiedKFold(n_splits=3, shuffle=True, random_state=42)

nested_scores = []
for train_idx, test_idx in outer_cv.split(X, y):
    X_train, X_test = X.iloc[train_idx], X.iloc[test_idx]
    y_train, y_test = y[train_idx], y[test_idx]

    grid = GridSearchCV(pipe, param_grid, cv=inner_cv, scoring='roc_auc', n_jobs=-1)
    grid.fit(X_train, y_train)
    score = grid.score(X_test, y_test)
    nested_scores.append(score)

print(f'Nested CV AUC: {np.mean(nested_scores):.3f} +/- {np.std(nested_scores):.3f}')

Stratified K-Fold

Goal: Evaluate model performance while preserving class proportions in each fold.

Approach: Split data into stratified folds and compute cross-validated scores to account for class imbalance.

from sklearn.model_selection import StratifiedKFold, cross_val_score

# Always stratify for class imbalance
cv = StratifiedKFold(n_splits=5, shuffle=True, random_state=42)
scores = cross_val_score(pipe, X, y, cv=cv, scoring='roc_auc')
print(f'CV AUC: {scores.mean():.3f} +/- {scores.std():.3f}')

Repeated Stratified K-Fold

Goal: Produce more stable performance estimates by averaging across multiple CV repetitions.

Approach: Repeat stratified K-fold splitting with different random seeds and aggregate scores across all iterations.

from sklearn.model_selection import RepeatedStratifiedKFold

# More robust estimate with multiple repeats
cv = RepeatedStratifiedKFold(n_splits=5, n_repeats=10, random_state=42)
scores = cross_val_score(pipe, X, y, cv=cv, scoring='roc_auc')
print(f'Repeated CV AUC: {scores.mean():.3f} +/- {scores.std():.3f}')

Leave-One-Out (Small Datasets)

Goal: Maximize training data when sample size is very small (n < 30).

Approach: Hold out one sample at a time for testing and train on all remaining samples, then aggregate predictions.

from sklearn.model_selection import LeaveOneOut, cross_val_predict

# Use for very small datasets (n < 30)
loo = LeaveOneOut()
y_pred = cross_val_predict(pipe, X, y, cv=loo, method='predict_proba')[:, 1]
auc = roc_auc_score(y, y_pred)
print(f'LOO AUC: {auc:.3f}')

Group-Aware Splits

Goal: Prevent data leakage when samples from the same patient or batch are correlated.

Approach: Use group-aware splitting to ensure all samples from a single group stay in the same fold.

from sklearn.model_selection import GroupKFold, LeaveOneGroupOut

# When samples from same patient/batch must stay together
groups = meta['patient_id'].values
group_cv = GroupKFold(n_splits=5)
scores = cross_val_score(pipe, X, y, cv=group_cv, groups=groups, scoring='roc_auc')

CV Strategy Selection

Dataset Size	Strategy	Notes
n > 100	StratifiedKFold(5)	Standard choice
n = 50-100	StratifiedKFold(10)	More train data per fold
n < 30	LeaveOneOut	Maximum train data
Repeated measures	GroupKFold	Keep patients together
High variance	RepeatedStratifiedKFold	More stable estimates

Avoiding Data Leakage

Goal: Ensure feature selection does not use test-fold information, which inflates performance estimates.

Approach: Embed feature selection inside a pipeline so it executes independently within each CV fold.

# WRONG: Feature selection before CV
# selected = SelectKBest(k=100).fit_transform(X, y)  # Leaks info!
# scores = cross_val_score(clf, selected, y, cv=cv)

# CORRECT: Feature selection inside CV
from sklearn.feature_selection import SelectKBest

pipe = Pipeline([
    ('scaler', StandardScaler()),
    ('select', SelectKBest(k=100)),  # Done per fold
    ('clf', RandomForestClassifier())
])
scores = cross_val_score(pipe, X, y, cv=cv, scoring='roc_auc')

Related Skills

• machine-learning/omics-classifiers - Model training
• experimental-design/multiple-testing - Multiple hypothesis correction
• machine-learning/biomarker-discovery - Feature selection within CV