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Date May 2021 Marks available 3 Reference code 21M.1.AHL.TZ2.13
Level Additional Higher Level Paper Paper 1 Time zone Time zone 2
Command term Estimate Question number 13 Adapted from N/A

Question

The graph below shows a small maze, in the form of a network of directed routes. The vertices A to F show junctions in the maze and the edges show the possible paths available from one vertex to another.

A mouse is placed at vertex A and left to wander the maze freely. The routes shown by dashed lines indicate paths sprinkled with sugar.

When the mouse reaches any junction, she rests for a constant time before continuing.

At any junction, it may also be assumed that

Determine the transition matrix for this graph.

[3]
a.

If the mouse was left to wander indefinitely, use your graphic display calculator to estimate the percentage of time that the mouse would spend at point F.

[3]
b.

Comment on your answer to part (b), referring to at least one limitation of the model.

[2]
c.

Markscheme

transition matrix is        M1A1A1


Note:
Allow the transposed matrix.
          Award M1 for a 6×6 matrix with all values between 0 and 1, and all columns (or rows if transposed) adding up to 1, award A1 for one correct row (or column if transposed) and A1 for all rows (or columns if transposed) correct.


[3 marks]

a.

attempting to raise the transition matrix to a large power             (M1)

steady state vector is 0.1570.08680.2560.2410.08680.173             (A1)

so percentage of time spent at vertex F is 17.3%                   A1


Note:
Accept 17.2%.


[3 marks]

b.

the model assumes instantaneous travel from junction to junction,             R1
and hence the answer obtained would be an overestimate             R1

OR

the mouse may eat the sugar over time             R1
and hence the probabilities would change             R1


Note: Accept any other sensible answer.


[3 marks]

c.

Examiners report

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

Syllabus sections

Topic 4—Statistics and probability » AHL 4.19—Transition matrices – Markov chains
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Topic 4—Statistics and probability

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