Quant Interview Sanity Checks
Quick sanity checks for quant interview answers, including range checks, units, edge cases, monotonicity, and payoff bounds.
Candidates who reach answers but sometimes miss impossible or implausible results.
Range checks
Probabilities must be between zero and one. Expected values should be plausible given the payoff range. Counts should not exceed the number of objects available. These checks are fast and catch severe mistakes.
Unit checks
Ask what units the answer has: probability, dollars, rolls, cards, seconds, or percentage points. If a fair price answer is dimensionless, or a probability has dollar units, the setup may be mixed.
Edge cases
Test simple extremes. If the number of trials is zero, what should happen? If probability is one, does the formula simplify correctly? Edge cases expose formulas copied without understanding.
Concrete example
For 1 - (5/6)^n, the probability of at least one six should be zero when n = 0 and should increase as n grows. If your expression decreases with more rolls, it fails a monotonicity check.
Payoff and decision checks
For betting questions, compare the expected value to possible payoffs and costs. A price above the maximum payoff or below the minimum payoff usually needs explanation or correction.
Common mistakes
Candidates often treat checking as optional cleanup after the answer. In interviews, checks are part of the answer because they show control over the model and help recover from small slips.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.