when r combines interaction and proc sas r does not fit multiple matrix what is sas vs r what is gls used for in r why does r complain about a singular matrix when i use sas why does sas complain about a singular matrix can r use sas mixed data what is the difference between r and sas difference between r and sas why does r complain about a singular matrix when it uses a mixed data in SAS? does r use a singular matrix in sas r gls definition in statistics what is delta in r what is r sas proc mixed what does the sas proc function do? why does r not run a multivariate model in sas why does r complain about a singular matrix how does r do an analysis does r use a singular matrix in sas what r is sas