次元削減

A dataset can have many input coordinates, including image measurements, word features, or sensor values. As dimension grows, representing a space densely can require much more data, and some distance-based procedures can become difficult to interpret. These are aspects of the curse of dimensionality rather than a theorem that every high-dimensional model fails. Dimensionality reduction creates fewer coordinates under a chosen objective. Principal component analysis, or PCA, finds orthogonal directions associated with high variance in centered data and projects onto selected directions. Each projected coordinate is a weighted linear combination of original features. High variance is not automatically useful target information, so discarding low-variance components can also discard predictive signals. PCA can reduce representation size and reconstruction error under its objective without guaranteeing better accuracy for every downstream classifier. The scikit-learn PCA transformation centers the input but does not scale each feature to unit variance by default. Those preprocessing choices matter when feature units differ. Nonlinear visualization methods such as t-SNE and UMAP emphasize neighborhood structure through different objectives and algorithms. UMAP constructs a representation based on a neighborhood graph and seeks a lower-dimensional embedding. A two-dimensional map can help explore groups, but distances, spacing, or density in that map need careful interpretation. The UMAP documentation specifically cautions that it does not preserve density well. Therefore a tightly packed visual cluster need not prove that the original points had the same density relationship. Validate patterns with the original data and the analysis task.