Day 5 of 133

A/B testing in production + DSA Two Pointers finish

MDE, sample size, SRM, peeking, CUPED — the four things that ruin tests.

DSA · NeetCode Two Pointers

Container With Most WaterDSA · Two Pointers
Interview questions to prep
1. Prove that moving the shorter pointer can never lose the optimal answer.
2. How is this different from trapping rain water, even though both look similar?
Trapping Rain WaterDSA · Two Pointers
Interview questions to prep
1. Compare DP (precompute left-max, right-max) vs two-pointer O(1) space — walk through the two-pointer invariant.
2. What if rainwater can flow off the sides — how does that change anything?

Experiment design: power, MDE, randomization, SRMStatisticsEvan Miller
Interview questions to prep
1. How do you choose sample size for an A/B test? What's MDE?
2. What is sample-ratio mismatch (SRM) and why is it a red flag?
3. Walk through how to design an experiment to detect a 1% lift in conversion.
Sequential / peeking problems & remediesStatisticsEvan Miller
Interview questions to prep
1. Why does 'checking the test daily and stopping when significant' inflate Type I error?
2. What are sequential testing methods (e.g., always-valid p-values) and when would you use them?
Variance reduction: stratification & CUPEDStatisticsMicrosoft
Interview questions to prep
1. Explain CUPED in plain English. Why does it shrink confidence intervals?
2. What's the difference between stratification and CUPED for variance reduction?

References & further reading