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Date	May 2022	Marks available	1	Reference code	22M.2.HL.TZ2.3
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Explain	Question number	3	Adapted from	N/A

Question

A group of students used quadrat sampling and the chi-squared test to find out whether the distributions of two plant species were associated with each other or not. These two species were found in the ground vegetation in a woodland ecosystem.

[Source: (left) Pixabay.
(right) Topic, J., n.d. Veronica montana 2. [image online] Available at: http://www.freenatureimages.eu/Plants/
Flora%20S-Z/Veronica%20montana/#Veronica%2520montana%25202%252C%2520Bosereprijs%252C%2520Saxifr
aga-Jasenka%2520Topic.jpg [Accessed 3 December 2019].

The numbers of quadrats with one, both or neither species present were counted and recorded. The observed frequencies from 150 quadrats are shown in the following contingency table.

State the alternative hypothesis for this study.

[1]

To calculate chi-squared, expected values must first be calculated. Assuming that there is no association between the two species, calculate the expected number of quadrats in which both species would be present, showing your working.

[1]

State the number of degrees of freedom for this test to determine the critical value of chi-squared.

[1]

When the data in the table were used to calculate chi-squared, the calculated value was 0.056. The critical value is 3.84. Explain the conclusion that can be drawn from the calculated and critical values for chi-squared.

[1]

Markscheme

there is a positive / negative association between the two species;
they tend to grow together / they tend to grow apart;

OWTTE

$\frac{70 \times 55}{150}$ ;
25.7;

Award [1] for proper values chosen/equation or [1] for answer.

1 (df)
OR
(r-1) (c-1);

(when the calculated value is smaller than the critical value) there is no significant association between the two species / H₀/null hypothesis accepted;
it is random chance if both species are either present or absent in most quadrats;

Examiners report

There were many correct answers, but a number confused the alternate and null hypotheses.

[N/A]

Calculating the degrees of freedom proved problematic for many.

[N/A]

Syllabus sections

Additional higher level » Topic 10: Genetics and evolution » 10.2 Inheritance

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Additional higher level » Topic 10: Genetics and evolution

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