Before revision:  Because the code is built around the reghdfe package, it has similar syntax and supports many of the same functionalities  . ppmlhdfe also implements a novel and more robust approach to check for the existence of (pseudo) maximum likelihood estimates.
After revision:  Because the code is built around the reghdfe package, it has similar syntax , supports many of the same functionalities . ppmlhdfe also implements a novel and more robust approach to check for the existence of (pseudo) maximum likelihood estimates.