Read the introduction (no need to read further) to the paper by Greenstone, Hornbeck & Moretti, Identifying Agglomeration Spillovers: Evidence from Winners and Losers of Large Plant Openings, published in the Journal of Political Economy in 2010.

It can be found here: https://eml.berkeley.edu//~moretti/mdp2.pdf

The authors compare two different types of counties in this paper to test whether or not the opening of a plant increases total factor productivity (TFP) of existing (incumbent) firms. In your own words, explain the difference between the two types of counties and how the authors selected them?

PS: It’s unnecessary to read slides and notes unless you need extra information.