When would you use clustering vs. regression on a business problem with partially labeled outcomes? Specify the decision criteria (label availability, objective, evaluation metrics, cost of errors). Enumerate at least four clustering algorithms (K-Means, Hierarchical/Agglomerative, DBSCAN/HDBSCAN, Gaussian Mixture Models) and compare assumptions, key hyperparameters, scalability, distance metrics, and failure modes (e.g., non-spherical clusters, varying density, high-dimensional sparsity, mixed data types). Give concrete scenarios selecting DBSCAN over K-Means and vice versa. Finally, explain K-Nearest Neighbors to a non-technical stakeholder with a real-world analogy, then deepen: choosing k, weighting by distance, effects of feature scaling, curse of dimensionality, and how to deploy KNN efficiently (KD-tree/ball-tree, approximate neighbors).