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Date May 2022 Marks available 2 Reference code 22M.1.SL.TZ1.7
Level Standard Level Paper Paper 1 Time zone Time zone 1
Command term State Question number 7 Adapted from N/A

Question

Leo is investigating whether a six-sided die is fair. He rolls the die 60 times and records the observed frequencies in the following table:

Leo carries out a χ2 goodness of fit test at a 5% significance level.

Write down the null and alternative hypotheses.

[1]
a.

Write down the degrees of freedom.

[1]
b.

Write down the expected frequency of rolling a 1.

[1]
c.

Find the p-value for the test.

[2]
d.

State the conclusion of the test. Give a reason for your answer.

[2]
e.

Markscheme

H0: The die is fair  OR  Pany number=16  OR  probabilities are equal

H1: The die is not fair  OR  Pany number16  OR  probabilities are not equal        A1

 

[1 mark]

a.

5      A1

 

[1 mark]

b.

10      A1

 

[1 mark]

c.

(p-value =0.287   0.28724163       A2

 

[2 marks]

d.

0.287>0.05         R1


EITHER

Insufficient evidence to reject the null hypothesis         A1


OR

Insufficient evidence to reject that the die is fair         A1

 

Note: Do not award R0A1. Condone “accept the null hypothesis” or “the die is fair”. Their conclusion must be consistent with their p-value and their hypothesis.

 

[2 marks]

e.

Examiners report

Many candidates confused this goodness of fit question with a χ2 test for independence and so incorrect statements for the hypotheses were seen frequently.

 

a.

Many candidates stated the degrees of freedom correctly.

 

 

b.

Expected value was not well understood. The most popular but erroneous answer was where the candidate calculated 860.

 

c.

Some confusion between the required p-value and the χ2 value. In the case of the latter, no further marks were available for this question. Too many candidates wrote down a value with p>1.

 

d.

Comparing their p-value with 0.05 or 5% correctly earned a mark for reasoning. Obtaining the reasoning mark enabled even those candidates with incorrect hypotheses in part (a), to be credited the final mark provided the conclusion was clear.

e.

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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