I am hoping the someone can answer a short question on ltm.
ltm installs successfully with
Error: package ‘polycor’ required by ‘ltm’ could not be found
If you are using linux, you should be able to type:
R --version into a terminal to confirm which version you have
edit: though it appears my R version is 3.2.3...I may need to change my ubuntu repo
sudo apt-cache showpkg r-base sudo apt-get upgrade r-base sudo apt-get install r-base