nick bloom, labor topics, spring 2010 labor topics nick bloom peers at work

Nick Bloom, Labor Topics, Spring 2010

LABOR TOPICS

Nick Bloom

Peers at Work


Comments on Mas and Moretti (2008)

• Great paper: innovative way to address an interesting topic – peer effects within workplaces• Huge dataset• High frequency productivity measurement• Interesting setting

Points to think about and learn from this I want to discuss:

(A) Reflection problem

(B) Bootstrap

(C) Presentation (graphics and robustness test)


Reflection problems (1/2)

One of the central issues in addressing spillovers is distinguishing these from:(A) Unobserved shocks(B) Sorting (selection effects)

These issues are often called the “Reflection Problem” after Manski (1993).

Ways to deal with this are:(A) Controlling for (instrumenting) unobserved shocks - Do this here using very long-run data, so no SR shocks

(B) Having random matching by pairs- Do this here by claiming the shift matching in random (and then test this)


Reflection problems (2/2)

(C) Using a distance metric to put more structure on the estimation - They do this with the facing/behind till distinction

Overall I think they do a convincing job of addressing the key issue in the “peer effects” and “spillovers” literature


Bootstrap and robustness tests

One issue they faced was in generating appropriate standard-errors in the regressions.

They had a generated regressor (predicted Θi) in the second step – this has error around it so needs its SE adjusted

Easiest way to do this is Bootstrap – keep re-drawing (with replacement) from the original data to look at the distribution of the coefficients.

- Idea is treats the sample as the population

Very computationally intensive (need to re-estimate everything 1000 times over) so they did a more complex Bayesian alternative


Presentation – graphics and robustness

They used fantastic graphics to prove their results – if you can always have some graphs of results, particularly with this kind of sharp discountinuity effect

They also always tested their key claims – for example that assignment of people to shifts is “random”

- If you ever make a claim in a paper always try to test this as much more convincing

nick bloom, labor topics, spring 2010 labor topics nick bloom peers at work

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