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June 2010 p63 q6
2497
The lengths of some insects of the same type from two countries, X and Y, were measured. The stem-and-leaf diagram shows the results.
Country X
Country Y
(10)
9 7 6 6 6 4 4 3 2 0
80
(18)
8 8 7 7 6 6 5 5 5 4 4 3 3 2 2 0
81
1 1 2 2 3 3 3 5 5 6 7 8 9
(13)
(16)
9 9 9 8 8 7 7 6 5 5 3 2 1 1 0 0
82
0 0 1 2 3 3 q 4 5 6 6 7 8 8
(15)
(16)
8 7 6 5 5 5 3 3 2 2 1 1 1 0 0
83
0 1 2 2 4 4 4 5 5 6 7 7 7 8 9
(17)
(11)
8 7 6 5 5 4 4 3 3 1 1
84
0 1 2 4 4 5 5 6 7 7 7 8 9
(15)
85
1 2 r 3 3 5 5 6 7 8 8
(12)
86
0 1 2 2 3 5 5 8 9 9
(11)
Key:5 | 81 | 3 means an insect from country X has length 0.815 cm and an insect from country Y has length 0.813 cm.
Find the median and interquartile range of the lengths of the insects from country X.
The interquartile range of the lengths of the insects from country Y is 0.028 cm. Find the values of \(q\) and \(r\).
Represent the data by means of a pair of box-and-whisker plots in a single diagram on graph paper.
Compare the lengths of the insects from the two countries.
Solution
(i) For Country X: the median is the middle ordered value; the interquartile range is \( \mathrm{IQR}=Q_3-Q_1 \).
From the mark scheme, \( \text{median}=0.825\text{ cm} \) and \( \mathrm{IQR}=0.019\text{ cm} \) (since \( Q_3=0.833 \) and \( Q_1=0.814 \)).
(ii) For Country Y, the given \( \mathrm{IQR}=0.028\text{ cm} \).
The missing values are \( q=4 \) and \( r=2 \).
(iii) Draw box-and-whisker plots for both countries with the correct medians and quartiles as in the mark scheme.
(iv) Comparison: Country Y’s insects are generally longer (higher median) and show a larger spread (greater range) than Country X’s.