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© Peter Broadfoot 2008
Histograms
Example 1 – Grouping Values
Here are eight values of a continuous variable. The variable could be any continuous
variable such as length or weight or time, and so it is represented by x.
Variable x
5.6
10.02
9.98
10
19.4
21.3
0.5
24.5
In the frequency distribution table below, complete column 2. Place each value in its
correct class. Record the frequency in column 3. This is an example question and so the
answers are already filled in (shown in blue).
Class Interval
Values of x
Frequency (count)
0 ≤ x < 10
0.5, 5.6, 9.98
3
10 ≤ x < 20
10, 10.02, 19.4
3
20 ≤ x < 30
21.3, 24.5
2
An Alternative Symbol for Class Interval
The hyphen symbol is sometimes used to show the class interval as in these examples:
Height h of Children in Centimetres
Class Interval
h (cm)
110
120
130
140
150
160
f
10
15
17
20
16
9
In the table the “110
" represents the class interval from height h=110cm up to, but not
including, 120cm. There are 10 children in that group (frequency f=10). An alternative is
to show 110
120 for the first class and then 120
130 for the next.
Weight of Children in kg
Class Interval
Weight (kg)
15.0
15.5
16.0
16.5
17.0
In this case the first class is any weight from exactly 15.0kg up to, but not including,
15.5kg, which belongs in the next class. 15.49 belongs in the first class. The last class
includes weights from exactly 17.0kg up to, but not including, 17.5kg.
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