Quant Interview Expected Value Mistakes
Common expected value mistakes in quant interviews, including confusing probability with value, over-enumerating, mishandling stopping rules, and ignoring risk.
Candidates practicing expected value, betting, fair-price, and stopping questions.
Confusing probability and value
Probability is likelihood. Expected value is average payoff. A 20 percent chance of winning 10 dollars has probability 0.2 and expected payoff 2 dollars. Keep the target and units separate.
Over-enumerating
Many EV questions become simple with linearity. If the question asks for an expected sum or count, look for indicators before enumerating every path.
Mishandling stopping rules
Stopping questions need states and boundary conditions. Candidates often use the average of a raw outcome when the continuation value is different in the current state.
Concrete example
In a one-reroll die game, the decision threshold is 3.5 because that is the value of rerolling. Keep 4, 5, and 6; reroll 1, 2, and 3. The threshold comes from continuation value.
Ignoring risk and size
Positive expected value does not automatically mean bet everything. Discuss variance, bankroll, constraints, and repeatability. Interviews often test decision quality, not just EV arithmetic.
Common mistakes
Candidates stop after the formula and skip the decision. A strong EV answer says what the value is, what assumptions created it, and how risk affects action.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.