Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
% cover
C.
vulgaris
below, there are two quadrats for each
species where this applies.
Next, calculate D (the difference between the
ranks). Then calculate D 2 for each species (as
in the χ 2 test this removes the negative signs).
Calculate the sum (Σ) of D 2 . (You could type
the table into a spreadsheet and get it to do
all the calculations for you.)
% cover
V.
myrtillus
Rank
C.
vulgaris
C.
vulgaris
D D 2
30 15 9 8 1 1
37 23 11 10 1 1
15 6 5.5 4 1.5 2.25
15 10 5.5 5.5 0 0
20 11 7 7 0 0
9 10 3 5.5 –2.5 6.25
3 3 1 2 –1 1
5 1 2 1 1 1
10 5 4 3 1 1
25 17 8 9 –1 1
35 30 10 11 –1 1
n = 11 ΣD 2 =
15.5
The figures are now inserted into the above
equation.
r s = 1 – 6 × ΣD2
n 3 – n
r s = 1 – 6 × 15.5
1331 – 11
r s = 1 – 0.070
r s = 0.930
A correlation coefficient of +0.93 is very close to
+1, so we can conclude that there is a positive
correlation between the two species and that
the strength of the association is very high.
Now look up the Spearman’s rank coefficient
in the table of critical values that correspond
to the number of pairs of measurements in
results table (there are 11).
Table showing the critical values of r s at p =
0.05 for different numbers of paired values
Number of pairs of
measurements
Critical value at p = 0.05
(5%)
5 1.00
6 0.89
7 0.79
8 0.74
9 0.68
10 0.65
11 0.60
12 0.59
13 0.56
14 0.54
15 0.52
16 0.51
17 0.49
18 0.48
19 0.45
20 0.44
30 0.36
If the value for r s is greater than the critical
value, then you can reject the null hypothesis.
If the value of r s is less than the critical value
you can accept the null hypothesis.
In this case the value for r s is greater than
the critical value so the ecologist can
reject the null hypothesis and accept the
alternative hypothesis.
d There is a significant correlation between
the abundance (as measured by percentage
cover) of the two species on the moorland.
e There are two variables: soil moisture and
percentage cover (or some other measurement
of abundance, such as species density)
Random sampling or systematic sampling
could be used in this investigation. This is the
method for random sampling:
Cambridge International AS and A Level Biology © Cambridge University Press 2014