This function is function is primarily used by [PSAboot::balance()] and probably does not need to be called directly.
balance.matching(index.treated, index.control, covs)a named vector with one element per covariate.
a vector with the index of treated rows in covs.
a vector with the index of control rows in covs.
data frame or matrix of covariates. Factors should already be recoded. See `[cv.trans.psa()]`