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Date November 2020 Marks available 1 Reference code 20N.1.SL.TZ0.T_10
Level Standard Level Paper Paper 1 Time zone Time zone 0
Command term Write down Question number T_10 Adapted from N/A

Question

On 90 journeys to his office, Isaac noted whether or not it rained. He also recorded his journey time to the office, and classified each journey as short, medium or long.

Of the 90 journeys to the office, there were 3 short journeys when it rained, 22 medium journeys when it rained, and 15 long journeys when it rained. There were also 14 short journeys when it did not rain.

Isaac carried out a χ2 test at the 5% level of significance on these data, looking at the weather and the types of journeys.

Write down H0, the null hypothesis for this test.

[1]
a.

Find the expected number of short trips when it rained.

[3]
b.

The p-value for this test is 0.0206.

State the conclusion to Isaac’s test. Justify your reasoning.

[2]
c.

Markscheme

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

type of journey and whether it rained are independent      (A1)   (C1)

Note: Accept “there is no association” or “not dependent”. Do not accept “not related” or “not correlated”. Accept equivalent terms for ‘type of journey’.


[1 mark]

a.

1790×4090×90  OR  17×4090      (A1)(M1)


Note:
Award (A1) for 17 or 40 seen. Award (M1) for 1790×4090×90  OR  17×4090 seen.


7.56  7.55555, 689      (A1)   (C3)


[3 marks]

b.

reject (do not accept) H0      (A1)

OR

type of journey and whether it rained are not independent      (A1)


Note: Follow through from part (a) for their phrasing of the null hypothesis.


0.0206<0.05      (R1)   (C2)


Note: A comparison must be seen, either numerically or in words (e.g. p-value < significance level). Do not award (R0)(A1).


[2 marks]

c.

Examiners report

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

Syllabus sections

Topic 4—Statistics and probability » SL 4.11—Expected, observed, hypotheses, chi squared, gof, t-test
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Topic 4—Statistics and probability

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