# An easier to use IV regression command in R

March 10, 2011
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(This article was first published on Coffee and Econometrics in the Morning, and kindly contributed to R-bloggers)

Update: I have added some functionality to my ivregress() command. Check out my newer post here.

After I posted my last video tutorial on how to use my IV regression function, I received a comment asking why I didn’t write the command a different way to make the syntax easier to read.

The answer is that I didn’t know how to write an easier to use function a year ago (when I wrote the ivreg() function). After some digging, I figured out how to work with “formula objects” in R and the result is an easier to use IV regression function (called ivregress()).

How to “install” ivregress()

Here’s the code you need to run to define ivregress() and its companion summary command sum.iv(). This will provide instrumental variables regression estimates if you have one endogenous regressor with one or more instruments.

You only need to run the above code once to define the function object ivregress() for all future uses (as long as you don’t write over it or clear your workspace).

How to use ivregress()

To create an “IV object” simply use the ivregress command as follows (the command relies on the car library):

library(car)
myivobject = ivregress(Y~X1+X2+…+Xk, Xj ~ Z1+Z2+…+Zl, dataframe)

where X1, X2,…Xk are second stage regressors, Xj is an endogenous regressor for which you would like to instrument, and Z1, Z2,…,Zl are instruments.

Then use sum.iv() from the previous posts to produce summary output. Sometime soon, I will post a video tutorial showing how to use ivregress() to perform IV regression.

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