Tightens an Observational Block Design by Balanced Subset Matching R package tightenBlock version 0.1.7 Tightens an observational block design into a smaller design with either smaller or fewer blocks while controlling for covariates. The method uses fine balance, optimal subset matching (Rosenbaum, 2012 <doi:10.1198/jcgs.2011.09219>) and two-criteria matching (Zhang et al 2023 <doi:10.1080/01621459.2021.1981337>). The main function is tighten(). The suggested 'rrelaxiv' package for solving minimum cost flow problems: (i) derives from Bertsekas and Tseng (1988) <doi:10.1007/BF02288322>, (ii) is not available on CRAN due to its academic license, (iii) may be downloaded from GitHub at <https://github.com/josherrickson/rrelaxiv/>, (iv) is not essential to use the package. Software Depends: R (>= 3.5.0) Imports: stats, MASS, rcbalance Suggests: rrelaxiv Paul Rosenbaum <rosenbaum@wharton.upenn.edu> Comprehensive R Archive Network (CRAN) Paul Rosenbaum [aut, cre] GPL-2 2023-12-15 application/tgz https://CRAN.R-project.org/package=tightenBlock doi:10.32614/CRAN.package.tightenBlock