Recent Posts

Here’s a puzzle for you. What will happen if we regress some outcome of interest on both an endogenous regressor and a valid instrument for that regressor? I hadn’t thought about this question until 2018, when one of my undergraduate students asked it during class.

R’s formula syntax is extremely powerful but can be confusing for beginners.1 This post is a quick reference covering all of the symbols that have a “special” meaning inside of an R formula: ~, +, .

To do well in an econometrics or statistics course at any level, you need to have a large number of simple properties of random variables at your fingertips. Some years back I made a handout containing the most important properties for my undergraduate students at the University of Pennsylvania.

In econometrics it’s absolutely crucial to keep track of which things are dependent and which are independent. To make this as confusing as possible for students, a typical introductory econometrics course moves back and forth between different notions of dependence, stopping occasionally to mention that they’re not equivalent but never fully explaining why, on the premise that “you’ve certainly already learned this in your introductory probability and statistics course.

The Poisson distribution is the most famous probability model for counts, non-negative integer values. Many real-world phenomena are well approximated by this distribution, including the number of German bombs that landed in 1/4km grid squares in south London during WWII.