Directional Distance Function for Efficiency/Productivity Analysis in Stata
net install ddfeff, from("https://raw.githubusercontent.com/kerrydu/ddfeff/master/")
Alternatively, download the zipfile and unzip it to your computer disk.
copy https://codeload.github.com/kerrydu/ddfeff/zip/master ddfeff-master.zip
unzipfile ddfeff-master.zip
net install ddfeff, from(`c(pwd)'/ddfeff-master)