IB Biology SL C4.1.15 Chi Squared Test Questions

Build hypotheses, expected frequencies and chi-squared conclusions from quadrat presence or absence data to test whether two species are associated in IB Biology SL C4.1.15.

Exam points

Use quadrat presence/absence data for two species to state null and alternative hypotheses about association and calculate expected frequencies.
Calculate or interpret chi-squared, degrees of freedom and critical values to decide whether observed species distributions show a significant association.

[Maximum number: 3]

White sage (Salvia apiana) is a native Californian shrub. Slender oat (Avena barbata) is a grass, originally from the Mediterranean, which was introduced to California. The map shows the distribution of the two species in relation to one another in an area near Santa Ynez, California.

Figure for Question IB Biology SL C4.1.15 Chi Squared Test Questions question 1 — IB Biology SL

Using the scale bar, determine the area of quadrat A1, giving the units.

Outline how chi-squared can be used to test for an association between the distributions of the two species.

in each quadrat determine the presence/absence «of plants» of each type
null hypothesis is that the presence of one is random/independent in relation to the presence of the other plant
OR
alternate hypothesis is that the presence of one is associated with the presence or absence of the other
$x^{2}=\sum \frac{(O-E)^{2}}{E}$
accept alternative hypothesis / reject null hypothesis if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2} /$ critical value
OR
it supports the association between the two species if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2} /$ critical value

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IB Biology SL C4.1.15 Chi Squared Test Questions question 1