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Date May 2022 Marks available 2 Reference code 22M.1.AHL.TZ2.12
Level Additional Higher Level Paper Paper 1 Time zone Time zone 2
Command term Find Question number 12 Adapted from N/A

Question

The sex of cuttlefish is difficult to determine visually, so it is often found by weighing the cuttlefish.

The weights of adult male cuttlefish are known to be normally distributed with mean 10kg and standard deviation 0.5kg.

The weights of adult female cuttlefish are known to be normally distributed with mean 12kg and standard deviation 1kg.

A zoologist uses the null hypothesis that in the absence of information a cuttlefish is male.

If the weight is found to be above 11.5kg the cuttlefish is classified as female.

90% of adult cuttlefish are male.

Find the probability of making a Type I error when weighing a male cuttlefish.

[2]
a.

Find the probability of making a Type II error when weighing a female cuttlefish.

[2]
b.

Find the probability of making an error using the zoologist’s method.

[2]
c.

Markscheme

P(Type I error) =P(stating female when male) 

=PWMale>11.5         (M1)

=0.00135  0.00134996          A1

 

[2 marks]

a.

P(Type II error) =P(stating male when female) 

=PWFemale<11.5         (M1)

=0.309   0.308537          A1

 

[2 marks]

b.

attempt to use the total probability           (M1)

P(error) =0.9×0.00134996+0.1×0.308537

=0.0321   0.0320687          A1

 

[2 marks]

c.

Examiners report

This was a straightforward problem on Type I and Type II errors which some candidates answered successfully in a couple of lines but many candidates were unable to do the correct calculations.

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

Syllabus sections

Topic 4—Statistics and probability » SL 4.9—Normal distribution and calculations
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