Distribution Comparison Through Density Ratio Estimation R package densityratio version 0.2.3 Fast, flexible and user-friendly tools for distribution comparison through direct density ratio estimation. The estimated density ratio can be used for covariate shift adjustment, outlier-detection, change-point detection, classification and evaluation of synthetic data quality. The package implements multiple non-parametric estimation techniques (unconstrained least-squares importance fitting, ulsif(), Kullback-Leibler importance estimation procedure, kliep(), spectral density ratio estimation, spectral(), kernel mean matching, kmm(), and least-squares hetero-distributional subspace search, lhss()). with automatic tuning of hyperparameters. Helper functions are available for two-sample testing and visualizing the density ratios. For an overview on density ratio estimation, see Sugiyama et al. (2012) <doi:10.1017/CBO9781139035613> for a general overview, and the help files for references on the specific estimation techniques. Software Depends: R (>= 2.10) Imports: osqp, Rcpp, pbapply, ggplot2, ggh4x LinkingTo: Rcpp, RcppArmadillo, RcppProgress Suggests: RcppArmadillo, knitr, rmarkdown, testthat (>= 3.0.0) Thom Volker <thombenjaminvolker@gmail.com> Comprehensive R Archive Network (CRAN) Thom Volker [aut, cre] (ORCID: <https://orcid.org/0000-0002-2408-7820>), Carlos Gonzalez Poses [ctb], Erik-Jan van Kesteren [ctb] GPL (>= 3) 2026-07-14 application/tgz https://CRAN.R-project.org/package=densityratio doi:10.32614/CRAN.package.densityratio