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>.