(string) single variable name that is passed by rtables when requested
by a statistics function.
.df_row
(data.frame) data set that includes all the variables that are called in .var and variables.
variables
(named list of string) list of additional analysis variables, with expected elements:
arm (string) group variable, for which the covariate adjusted means of multiple groups will be
summarized. Specifically, the first level of arm variable is taken as the reference group.
covariates (character) a vector that can contain single variable names (such as "X1"), and/or
interaction terms indicated by "X1 * X2".
interaction_item
(string or NULL) name of the variable that should have interactions
with arm. if the interaction is not needed, the default option is NULL.