Conditional density estimation generalizes regression by modeling a full density f(yjx) rather than only the expected value E(yjx). This is important for many tasks, including handling multi-modality and generating prediction intervals. Though fundamental and widely applicable, nonparametric conditional density estimators have received relatively little attention from statisticians and little or none from the machine learning community. None of that work has been applied to greater than bivariate data, presumably due to the computational difficulty of data-driven bandwidth selection. We describe the double kernel conditional density estimator and derive fast dual-tree-based algorithms for bandwidth selection using a maximum likelihood criterion. These techniques give speedups of up to 3.8 million in our experiments, and enable the first applications to previously intractable large multivariate datasets, including a redshift prediction problem from the Sloan Digital Sky Survey.

通过建立 full density 模型 f(yjx) 而非只有期望值 E(yjx)，条件密度估计广义了回归。本文提出了双核条件密度估计器，并引入了基于双数树的快速算法，用最大似然准则进行带宽选择，从而在处理多变量数据集时取得 380 万倍的加速。

快速非参数条件密度估计