6. Dataset transformations 6.1.1. Pipeline: chaining estimators6.1.3. FeatureUnion: composite feature spaces 6.2. Feature extraction 6.2.2. Feature hashing6.2.4. Image feature extraction 6.3.1. Standardization, or mean removal and variance scaling6.3.3. Normalization6.3.5. Discretization6.3.8. Custom transformers 6.4.1. Univariate vs. Multivariate Imputation6.4.3. Multivariate feature imputation6.4.5. Nearest neighbors imputation 6.5. Unsupervised dimensionality reduction 6.5.2. Random projections 6.6. Random Projection 6.6.2. Gaussian random projection 6.7.1. Nystroem Method for Kernel Approximation6.7.3. Additive Chi Squared Kernel6.7.5. Mathematical Details 6.8.1. Cosine similarity6.8.3. Polynomial kernel6.8.5. RBF kernel6.8.7. Chi-squared kernel 6.9.1. Label binarization