Session 6: Practice

Does your finding survive the structure of your data?

Thierry Warin, PhD

HEC Montréal

Outline · The workshop

  1. Ninety minutes
  2. Before you start
  3. Reproduce — 20 min
  4. Apply to your own angle — 35 min
  5. Break one assumption — 15 min
  6. Write it down — 20 min
  7. Submitting
  8. What loses marks
  9. Two-minute report
  10. After class, if you want it

What session 6 is for

The mathematics

  • Summarise sessions 2 to 5 as goal, implementation, results and interpretation
  • State what a panel is, and distinguish fixed from random effects
  • Use the Hausman test to choose between fixed and random effects
  • Read an interaction as a slope that differs between groups
  • Read a squared term as an effect that accelerates or levels off

The Python

  • Fit fixed- and random-effects models with PanelOLS and RandomEffects
  • Write an interaction (a * b) and a quadratic (I(x**2)) in a formula
  • Interpret both in units, from the fitted coefficients

The international business

  • Separate within-country change from between-country difference
  • Say which specification your identifying variation can actually support
  • Report the estimate and the specification, because neither means much alone

Your Python so far

Everything you need to type is on one sheet — how to start Python in VS Codium on your platform, how to make a file and run it, the DataFrame basics, and every name from sessions 1 to 6, each with a line showing it in use.

Python Cheatsheet · Session 06 (PDF) · in the repository at 06-…/PYTHON-CHEATSHEET.pdf

One sheet per package, functions explained with pictures: pandas · NumPy · matplotlib · statsmodels · linearmodels · scikit-learn · SciPy

Print it, or keep it open beside the editor. Nothing on it is examinable; all of it is assumed.

Practice

Does your finding survive the structure of your data?

Ninety minutes

00–20 Reproduce one result from the paper you read
20–55 Apply the method, to your own angle
55–70 Break one assumption, deliberately
70–90 Write, and report 250 words, then two minutes to the room

The presenter is drawn at random when your group is called. All three of you prepare.

Before you start

Everyone pushes at least once. Replace XX with your group number — group 07 uses group-07.

git checkout -b group-XX
mkdir -p groups/A2026/group-XX/session-06      # macOS and Linux
git checkout -b group-XX
mkdir groups/A2026/group-XX/session-06         # Windows — no -p

PowerShell’s mkdir creates the intermediate folders already, and rejects -p. The git log is the participation record. It is not a formality.

1 · Reproduce — 20 min

Take the result the paper rests on, and reproduce it. The package you downloaded is at 06-advanced-regression/data/replication/.

import qmib

data = qmib.load("panel")     # the session's own data

Paper: Topalova & Khandelwal (2011), Trade Liberalization and Firm Productivity · 10.7910/DVN/8WEXYD

A failed reproduction that you diagnose earns full marks. One you do not attempt earns none.

“It did not work” is not a diagnosis. Where did it stop, and what did you check?

2 · Apply to your own angle — 35 min

core = qmib.load("core")
mine = qmib.load("angle_c_country")     # your angle — see your dictionary
df   = core.merge(mine, on=["geo", "time"], how="inner")

Fit the session’s method on your data, and report what the lecture said to report — in the units of your own variables.

A coefficient without units is not a finding.

3 · Break one assumption — 15 min

Every method in this course rests on something.

Find the assumption this one leans on hardest. Break it deliberately. Record what happened to your answer.

This is the part that carries the marks. A group that shows how its result falls apart understands it; a group that only shows it holding does not yet know.

4 · Write it down — 20 min

250 words in your submissions folder:

  • What you found, in units
  • Which assumption you broke, and what it did
  • What this result does not license you to claim

And the standing rule: one LLM output you identified as wrong, and how you established that.

Submitting

git add groups/A2026/group-XX/session-06
git commit -m "Session 06 practice — group XX"
git push -u origin group-XX

Deliverable. A panel specification of your project’s core relationship, fitted with both fixed and random effects, plus a 250-word note on which you would report and why.

What loses marks

  • Pooling a panel with ordinary standard errors
  • Choosing fixed or random effects because one gave the significant answer
  • Reporting an interaction without stating the slope in each group
  • Adding a quadratic term and interpreting only its coefficient

And everywhere in this course: an association described in causal language.

Two-minute report

One number with its units and its uncertainty
One assumption the one you broke, and what happened
One limit what you cannot claim from this

Listen for traceable evidence, not confidence of delivery.

After class, if you want it

The QMIB Lab has a knowledge check on what you have just done: warin.ca/qmib-labs

Optional, never a prerequisite for the next lecture, and never marked for correctness. A completed report counts because attempting it is the engagement being measured — it is one of the three routes to the participation mark.

What is required before the next session: the article, its Dataverse package, and the self-check. That is the pre-session deck, not this.