do you need a missing variable in matchit can you use matchit to match a sample can matchit be used in regression analysis? what is the missing variable in matchit when to use matchit in retrospective analysis why can't matchit be used for a treatment can you use match it with a predefined treatment variable is match it supported in retrospectives can matchfit use missed variables how does matchit use missing variables is matchit missing in a regression study? does match it support missing variables what is the problem with matchit why do you use matchit can matchit detect missing variable matchit missing variables which r package does matchit use what is the purpose of a matchit how many observations are in the treatment group? can i use matchit for group matching