what is the difference between the variable and the variable variable what is the difference between r and xtreg what is the difference between r and xtreg? what is the difference between xtreg and plm? is gamma_i a variable in stata difference between r and r xtreg difference between fixed effects and stata what is the difference between xtreg and r difference between r and stata what is the difference between a fixed and independent model? what model a gamma_t and delta_t what is the difference between r model xtreg and r set model xtreg difference between sql and r the difference between stata and r difference between linear model and r what is the difference between stata and plm difference between stata and r what is the difference between r and a fixed effect model difference between stata and model difference in r and xtreg