DP Biology (first assessment 2025)

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Question 22M.2.HL.TZ2.3

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Date May 2022 Marks available [Maximum mark: 4] Reference code 22M.2.HL.TZ2.3
Level HL Paper 2 Time zone TZ2
Command term Calculate, Explain, State Question number 3 Adapted from N/A
3.
[Maximum mark: 4]
22M.2.HL.TZ2.3

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.

(a)

State the alternative hypothesis for this study.

[1]

Markscheme

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

OWTTE

Examiners report

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

(b)

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]

Markscheme
  1. 70 × 55150 ;
  2. 25.7;

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

(c)

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

[1]

Markscheme

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

Examiners report

Calculating the degrees of freedom proved problematic for many.

(d)

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

Syllabus sections