Difference-in-differences for product and policy changes

Difference-in-Differences
Quasi-Experiments
Panel Data
A concise note on using pre/post comparisons with treated and comparison groups.
Published

April 26, 2026

Core Idea

Difference-in-differences compares changes over time in a treated group to changes over time in a comparison group.

What Makes It Credible

  • Treated and comparison units had similar pre-treatment trends.
  • The timing of treatment was not driven by an unobserved shock that also changed the outcome.
  • There are enough pre-periods to inspect trends.
  • The intervention did not spill over into the comparison group.

Industry Uses

  • City or market rollouts.
  • Pricing or promotion changes.
  • Product launches with staggered adoption.
  • Policy changes affecting selected units.