Partitioned Cross-Validation for Divide-and-Conquer Density Estimation
  We present an efficient method to estimate cross-validation bandwidth parameters for kernel density estimation in very large datasets where ordinary cross-validation is rendered highly inefficient, both statistically and computationally. Our approach relies on calculating multiple cross-validation bandwidths on partitions of the data, followed by suitable scaling and averaging to return a partitioned cross-validation bandwidth for the entire dataset. The partitioned cross-validation approach produces substantial computational gains over ordinary cross-validation. We additionally show that partitioned cross-validation can be statistically efficient compared to ordinary cross-validation. We derive analytic expressions for the asymptotically optimal number of partitions and study its finite sample accuracy through a detailed simulation study. We additionally propose a permuted version of partitioned cross-validation which attains even higher efficiency. Theoretical properties of the estimators are studied and the methodology is applied to the Higgs Boson dataset with 11 million observations 
Anirban Bhattacharya, Jeffrey D. Hart
stat.ME