Before revision:  Since 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 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.