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Natural strata fix a constant ratio of controls to treated units within each stratum. This ratio need not be an integer. The control units are chosen using randomized rounding of a linear program that balances many covariates. To solve the linear program, the 'Gurobi' commercial optimization software is recommended, but not required. The 'gurobi' R package can be installed following the instructions at <https://www.gurobi.com/documentation/9.1/refman/ins_the_r_package.html>.
Package details 


Author  Katherine Brumberg [aut, cre] 
Maintainer  Katherine Brumberg <kbrum@wharton.upenn.edu> 
License  GPL3 
Version  1.0.0 
URL  https://github.com/kkbrum/natstrat https://kkbrum.github.io/natstrat/ https://www.gurobi.com/documentation/9.1/refman/ins_the_r_package.html 
Package repository  View on CRAN 
Installation 
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