Probability Interview Questions for Quant Roles
How to prepare for quant probability interview questions covering coins, dice, cards, Bayes, random walks, urns, and expected value.
Candidates who need a probability-first quant interview plan.
What probability interviews test
Probability rounds test whether you can choose a model before calculating. Coins usually test independence and stopping rules, dice test counting, cards test sampling without replacement, Bayes questions test base rates, and random walks test state recursion.
Concrete example
If a fair coin is flipped until the first head, the expected number of flips is 2. You can derive it with E = 1 + 0.5E, or by summing n/2^n. The useful habit is naming the recursive state before moving symbols around.
How to practice
Build a method checklist: complement counting, conditioning on the first step, linearity of expectation, symmetry, Bayes rule, and recursion. After each problem, write the one sentence that would have identified the right method faster.
Common mistakes
The main failure mode is treating every probability problem as a formula lookup. Another is assuming independence when draws are without replacement. Slow down for the setup, then speed up the arithmetic once the structure is correct.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.