Added Authors@R field to DESCRIPTION to address the CRAN check NOTE.
Added the DOI for the paper proposing the corrected sandwich variance method.
Added support for estimating the average treatment effect in the treated (ATT) using ATT weights.
Added a check to identify and remove covariates with the same value across all observations, as they do not contribute to propensity score estimation.
Added an error message for missing data.
Updated the package and function descriptions.
Generalized the code to handle categorical and factor covariates directly, eliminating the need for users to convert them to dummy variables before analysis.
Simplified some matrix calculation code.