Find the Expected Number of Record Maxima

Quick Overview

A permutation of the distinct values `1` through `100` is revealed from left to right. Explain the assumptions and derivation clearly, check edge cases, and show how the result changes when those assumptions no longer hold.

Find the Expected Number of Record Maxima

Company: Worldquant

Role: Data Scientist

Category: Statistics & Math

Difficulty: medium

Interview Round: Technical Screen

A permutation of the distinct values `1` through `100` is revealed from left to right. Replace the current maximum whenever the revealed value is strictly greater than every earlier value. 1. What is the expected number of replacements? 2. Repeat the calculation when the shuffled multiset is `1, 2, ..., 98, 100, 100`, where a second copy equal to the current maximum does not count as a replacement. ### Constraints & Assumptions - Every ordering of the distinct list is equally likely. - For the multiset, start from a uniformly random permutation of labeled copies and then ignore the labels on the two `100` values. - The first revealed value counts as a replacement. ```hint Use indicator variables For each value, ask whether it appears before every strictly larger value. ``` ### What a Strong Answer Covers - Linearity of expectation without requiring record events to be independent. - The harmonic-number result for distinct values. - Correct treatment of the duplicated maximum and strict comparison in the follow-up. ### Follow-up Questions - What is the asymptotic growth of the expectation? - How would a replace-on-greater-or-equal rule change the duplicate case? - What is the probability of exactly one replacement in the distinct case?

Quick Answer: A permutation of the distinct values `1` through `100` is revealed from left to right. Explain the assumptions and derivation clearly, check edge cases, and show how the result changes when those assumptions no longer hold.

|Home/Statistics & Math/Worldquant
Worldquant logo
Worldquant
Jan 5, 2025, 12:00 AM
mediumData ScientistTechnical ScreenStatistics & Math
0
0

A permutation of the distinct values 1 through 100 is revealed from left to right. Replace the current maximum whenever the revealed value is strictly greater than every earlier value.

  1. What is the expected number of replacements?
  2. Repeat the calculation when the shuffled multiset is 1, 2, ..., 98, 100, 100 , where a second copy equal to the current maximum does not count as a replacement.

Constraints & Assumptions

  • Every ordering of the distinct list is equally likely.
  • For the multiset, start from a uniformly random permutation of labeled copies and then ignore the labels on the two 100 values.
  • The first revealed value counts as a replacement.

What a Strong Answer Covers Guidance

  • Linearity of expectation without requiring record events to be independent.
  • The harmonic-number result for distinct values.
  • Correct treatment of the duplicated maximum and strict comparison in the follow-up.

Follow-up Questions Guidance

  • What is the asymptotic growth of the expectation?
  • How would a replace-on-greater-or-equal rule change the duplicate case?
  • What is the probability of exactly one replacement in the distinct case?
Loading comments...