This week, we’re going to think about non-experimental study design and about interpreting results of statistical analysis/hypothesis testing.

Discuss the following questions as a group. Nominate a member of your group to communicate your thoughts/conclusions to the class as a whole when we reconvene.

Modeling air quality monitor placement

Two economists, Grainger and Schreiber, built a model of the probability that an air quality monitor is placed in a specific site. A “site” in their study is a 0.05x0.05 degree grid cell—basically, they cut the country up into a bunch of tiny boxes and model the probability an air quality monitor gets put into any specific box.

They model this probability as a function of many factors: air pollution levels, the demographics of the box, home prices in the box, whether the county has been designated as “non-attainment” (has excess measured air pollution levels, in violation of the relevant regulations), and idiosyncratic spatial and time-varying effects (the “fixed effects”).

A note on terminology: “indicator” variables are discrete variables which are either 0 or 1. For example, “Attainment” is an indicator variable which is 1 if the county is “in attainment” (not in violation) or 0 if the county is “in non-attainment” (in violation).



1. What might indicate whether air quality monitor siting decisions are systematically biased?

  1. Suppose you had 10 pollution monitors to cover a 100 square km region. How would you place them to ensure representative sampling of air quality?
  2. How would you place them if you wanted to
    1. protect certain groups more than others?
    2. avoid detecting high pollution levels?
  3. What did the authors look for as evidence of strategic monitor siting?


2. What do the authors find?

For these questions, refer to figure 1 of the paper.

  1. What does the y-axis measure? (Hint: Don’t just read the axis!)
  2. Some of the error bars on their estimates include zero, and some don’t. Using the language of hypothesis testing: what does this mean?