Quant Interview Probability Mistakes
Common quant interview probability mistakes, including wrong sample spaces, independence assumptions, conditioning errors, and ordered-count confusion.
Candidates missing probability prompts despite knowing the formulas.
Wrong sample space
The most common probability mistake is counting in the wrong universe. Before calculating, define one outcome and the total set of outcomes. This prevents many denominator errors.
False independence
Draws without replacement, revealed information, and shared constraints create dependence. If the probability of the next event changes after the previous event, independence is not available.
Conditioning errors
Conditional probability asks for a fraction inside the given information. Candidates often keep the old denominator or swap P(A | B) with P(B | A). Use counts when the wording gets slippery.
Concrete example
If a family has two children and at least one is a boy, the standard ordered model leaves BB, BG, and GB. The chance both are boys is 1/3, not 1/2, because the information changed the sample space.
Ordered versus unordered confusion
Dice rolls are often ordered; card hands are often unordered. Both approaches can work if numerator and denominator match. Mixing them creates wrong answers that may look plausible.
Common mistakes
Candidates sometimes memorize formulas to avoid setup. The fix is the opposite: slow down for sample space, independence, conditioning, and order before using any formula.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.