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Date May Example question Marks available 5 Reference code EXM.1.SL.TZ0.8
Level Standard Level Paper Paper 1 Time zone Time zone 0
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Question

In an effort to study the level of intelligence of students entering college, a psychologist collected data from 4000 students who were given a standard test. The predictive norms for this particular test were computed from a very large population of scores having a normal distribution with mean 100 and standard deviation of 10. The psychologist wishes to determine whether the 4000 test scores he obtained also came from a normal distribution with mean 100 and standard deviation 10. He prepared the following table (expected frequencies are rounded to the nearest integer):

 

Copy and complete the table, showing how you arrived at your answers.

[5]
a.

Test the hypothesis at the 5% level of significance.

[6]
b.

Markscheme

To calculate expected frequencies, we multiply 4000 by the probability of each cell:

   p(80.5X90.5)=p(80.510010Z90.510010)       (M1)

          =p(19.5Z0.95)

          =0.17110.0256

          =0.1455

Therefore, the expected frequency =4000×0.1455       (M1)

          582       (A1)

Similarly: p(90.5X100.5)=0.51990.1711

          =0.3488

    Frequency =4000×0.3488

          1396       (A1)

And p(100.5X110.5)=0.85310.5199

          =0.3332

    Frequency =4000×0.3332

          1333       (A1)

[5 marks]

a.

To test the goodness of fit of the normal distribution, we use the χ2 distribution. Since the last cell has an expected frequency less than 5, it is combined with the cell preceding it. There are therefore 7 – 1 = 6 degrees of freedom.          (C1)  

χ2=(206)26+(9096)296+(575582)2582+(12821396)21396+(14501333)21333+(499507)2507+(8480)802            (M1)

= 53.03          (A1)          

H0: Distribution is Normal with μ=100 and σ=10.

H1: Distribution is not Normal with μ=100 and σ=10.            (M1)

χ2(0.95,6)=14.07

Since χ2=53.0>χ2critical=14.07, we reject H0          (A1) 

Or use of p-value

Therefore, we have enough evidence to suggest that the normal distribution with mean 100 and standard deviation 10 does not fit the data well.          (R1) 

Note: If a candidate has not combined the last 2 cells, award (C0)(M1)(A0)(M1)(A1)(R1) (or as appropriate).

[6 marks]

b.

Examiners report

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

Syllabus sections

Topic 4—Statistics and probability » SL 4.9—Normal distribution and calculations
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Topic 4—Statistics and probability » SL 4.11—Expected, observed, hypotheses, chi squared, gof, t-test
Topic 4—Statistics and probability

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