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Date November 2016 Marks available 3 Reference code 16N.2.AHL.TZ0.H_3
Level Additional Higher Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number H_3 Adapted from N/A

Question

A discrete random variable X follows a Poisson distribution Po ( μ ) .

Show that P ( X = x + 1 ) = μ x + 1 × P ( X = x ) ,   x N .

[3]
a.

Given that P ( X = 2 ) = 0.241667  and P ( X = 3 ) = 0.112777 , use part (a) to find the value of μ .

[3]
b.

Markscheme

* This question is from an exam for a previous syllabus, and may contain minor differences in marking or structure.

METHOD 1

P ( X = x + 1 ) = μ x + 1 ( x + 1 ) ! e μ    A1

= μ x + 1 × μ x x ! e μ    M1A1

= μ x + 1 × P ( X = x )    AG

METHOD 2

μ x + 1 × P ( X = x ) = μ x + 1 × μ x x ! e μ    A1

= μ x + 1 ( x + 1 ) ! e μ    M1A1

= P ( X = x + 1 )    AG

METHOD 3

P ( X = x + 1 ) P ( X = x ) = μ x + 1 ( x + 1 ) ! e μ μ x x ! e μ    (M1)

= μ x + 1 μ x × x ! ( x + 1 ) !    A1

= μ x + 1    A1

and so P ( X = x + 1 ) = μ x + 1 × P ( X = x )      AG

[3 marks]

a.

P ( X = 3 ) = μ 3 P ( X = 2 )   ( 0.112777 = μ 3 0.241667 )    A1

attempting to solve for μ      (M1)

μ = 1.40    A1

[3 marks]

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Topic 4—Statistics and probability » SL 4.8—Binomial distribution
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Topic 4—Statistics and probability » AHL 4.17—Poisson distribution
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