Zelig has recently been updated and does not work with our scripts. If you've installed the new version by mistake, follow the instructions below to go back to the old version 4.2.1:
First remove the package
remove.packages("Zelig")
Close RStudio and open it again.
Install the package
install.packages("https://cran.r-project.org/src/contrib/Archive/Zelig/Zelig_4.2-1.tar.gz",
repos=NULL,
type="source")
If you've never installed Zelig before then you might get an error about missing dependency for package sandwich. If that happens then install sandwich first and repeat the previous step.
install.packages("sandwich")
If you already have Zelig installed, you can check its version using the packageVersion() function. Make sure it says version 4.2.1.
packageVersion("Zelig")
[1] '4.2.1'