Probability · univariate distributions diagnostic PDF

Exam P Univariate Distributions: 30 Worked Problems

Practice model selection, parameter conventions, moments, quantiles, transformations, and insurance-payment variables across the univariate syllabus.

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Version 1.0.0 · Updated 2026-08-24

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One original Exam P practice question with full solution

A quality score takes values 1, 3, 5, 7, and 9 with probabilities 0.10, 0.20, 0.40, 0.20, and 0.10. What probability lies within one standard deviation of its mean, including endpoints?

  1. 0.20
  2. 0.40
  3. 0.60
  4. 0.80
  5. 1.00

Symmetry gives mean 5 and variance 4.8, so the interval is about [2.81,7.19]. Scores 3, 5, and 7 contribute probability 0.80.

Setup

Use symmetry of the five-point score distribution to obtain mean 5, then compute its variance from squared deviations about 5.

E[S]=5E[S]=5
E[(S5)2]=0.1(16)+0.2(4)+0.4(0)+0.2(4)+0.1(16)=4.8E[(S-5)^2]=0.1(16)+0.2(4)+0.4(0)+0.2(4)+0.1(16)=4.8

Model

The variance is 4.8, giving standard deviation 2.1909 and interval [2.8091,7.1909].

σS=4.8=2.1909\sigma_S=\sqrt{4.8}=2.1909
2.8091S7.19092.8091\le S\le7.1909

Compute

Only scores 3, 5, and 7 lie in the interval. Their probabilities sum to 0.20+0.40+0.20=0.80.

S{3,5,7}S\in\{3,5,7\}
P=0.20+0.40+0.20=0.80P=0.20+0.40+0.20=0.80

Answer

Hence the required probability is 0.80, corresponding to choice D.

0.80(D)\boxed{0.80\quad\text{(D)}}

Why the other choices are wrong

A
The value 0.20 counts only one of the two symmetric neighboring scores and omits both the center and the other neighbor.
B
The value 0.40 is the mass at score 5 alone; scores 3 and 7 also lie within one standard deviation.
C
The value 0.60 includes the center and only one of the two symmetric neighboring scores.
E
The value 1.00 includes scores 1 and 9, each four units from the mean and therefore outside the radius 2.1909.

Independent answer check

Primary value0.8
Primary methodCompute the variant by direct discrete moment calculation and support filtering.
Independent value0.8
Independent methodComputing each absolute deviation from 5 directly includes 3 and 7 but excludes 1 and 9, giving 0.80.
AnswerD · agreement passed

Contents

Inside the 76-page edition

Candidates preparing in English for the Probability (P) Exam

Table of contents

  1. 30 original univariate problems
  2. Discrete and continuous model selection
  3. Moments, percentiles, and transformations
  4. Deductibles and payment variables
  5. 120 wrong-choice analyses and two-path checks

What is included

30 verified original variants selected without duplication for this focused diagnostic. Official problem wording is not included.

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Questions about this edition

Are these official sample questions?

No. Every question is an independently written original variant. Official problem wording is not reproduced.

Are complete solutions included?

Yes. Every problem includes Setup, Model, Compute, and Answer steps plus a concrete explanation for each of the four incorrect choices.

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Yes. These 30 original variants are selected from the verified variant bank included in the Probability Proof Manual. This edition adds a curated focused diagnostic, answer sheet, answer key, and focused purchase price.

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Probability Univariate Distributions Practice Workbook

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