Field note · November 16, 2025
Correction: Difference-in-differences
The page claimed more than it should have. What it says now, and where the pattern stops applying.
Sharpened the wording on Difference-in-differences after someone pointed out it was overstated.
The original read as though the pattern always applies. It does not. When a change hits one group and not another, compare the change over time rather than the levels. Any time-invariant difference cancels out.
What it should have said — and now does — is where it stops applying. Every technique has a range of validity, and a pattern library that states rules without stating their limits produces exactly the cargo-culting it was meant to prevent.
The revised page names the case where the simple version is fine, so you are not paying the cost of the careful version on a table with four rows.
Corrections go in the page, with a note about what changed. No silent edits. The course will contain errors — it is thirty-six lessons about a field where the standard advice is frequently wrong — and the only defensible approach is to fix them visibly.
The repository takes issues, and several pattern pages are sharper than what we shipped because someone argued.