How do you measure impact when you can't run a clean A/B test (difference-in-differences, synthetic control, IV)?

Senior DS loops test causal reasoning past the randomized A/B test. The signal is naming the quasi-experimental method that fits and stating the single assumption it lives or dies on. Here is the answer for when randomization is off the table.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.