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Estimate total attendance from size-biased reservation sample

Last updated: Mar 29, 2026

Quick Overview

This question evaluates understanding of size-biased sampling, selection bias correction, finite-population total estimation, and uncertainty quantification in survey sampling.

  • easy
  • Waymo
  • Statistics & Math
  • Data Scientist

Estimate total attendance from size-biased reservation sample

Company: Waymo

Role: Data Scientist

Category: Statistics & Math

Difficulty: easy

Interview Round: Technical Screen

You run a restaurant with **N = 10,000 reservations** in a day. Each reservation *j* has: - Reserved party size: \(R_j\) (positive integer) - Actual number of people who show up: \(S_j\) (can be \(<\), \(=\), or \(>\) \(R_j\)) To reduce measurement effort, you only observe \((R,S)\) for a **sample of n = 1,000 reservations**. The sampling is **size-biased**: - A reservation with reserved size 10 is **twice as likely** to be sampled as a reservation with reserved size 5. - More generally, assume the sampling probability is **proportional to reserved size**: \(\Pr(\text{reservation } j \text{ is sampled}) \propto R_j\). ### Task Using the 1,000 sampled pairs \((R_i,S_i)\), estimate **the total number of people who would show up across all 10,000 reservations**: \[ T_S = \sum_{j=1}^{10000} S_j. \] State any assumptions needed, and explain how you would quantify uncertainty (e.g., a confidence interval).

Quick Answer: This question evaluates understanding of size-biased sampling, selection bias correction, finite-population total estimation, and uncertainty quantification in survey sampling.

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Waymo
Nov 11, 2025, 12:00 AM
Data Scientist
Technical Screen
Statistics & Math
17
0

You run a restaurant with N = 10,000 reservations in a day. Each reservation j has:

  • Reserved party size: RjR_jRj​ (positive integer)
  • Actual number of people who show up: SjS_jSj​ (can be <<< , === , or >>> RjR_jRj​ )

To reduce measurement effort, you only observe (R,S)(R,S)(R,S) for a sample of n = 1,000 reservations. The sampling is size-biased:

  • A reservation with reserved size 10 is twice as likely to be sampled as a reservation with reserved size 5.
  • More generally, assume the sampling probability is proportional to reserved size : Pr⁡(reservation j is sampled)∝Rj\Pr(\text{reservation } j \text{ is sampled}) \propto R_jPr(reservation j is sampled)∝Rj​ .

Task

Using the 1,000 sampled pairs (Ri,Si)(R_i,S_i)(Ri​,Si​), estimate the total number of people who would show up across all 10,000 reservations:

TS=∑j=110000Sj.T_S = \sum_{j=1}^{10000} S_j.TS​=∑j=110000​Sj​.

State any assumptions needed, and explain how you would quantify uncertainty (e.g., a confidence interval).

Solution

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