We will start by looking at a graphical
method for studying the variation known
as the 'Frequency Histogram'.
To create a frequency histogram, group
the data into ‘bins’, each bin containing
a range of values. The data below show
the test results for 25 students:
Results 

Bin 
Frequency 
38

10 
60 
90 
88 

>020

7 
96 
1 
41 
86 
14 

>2040

8 
25 
5 
3 
16 
22 

>4060

5 
2 
29

34 
55 
36


>6080

0 
37 
36 
91 
47 
43 

>80100

5 
I've grouped them into 5 bins of equal
span. The first bin contains the frequency
(or number) of results that are greater
than zero and up to and including 20.
The second bin contains the frequency
of values greater than 20 up to and including
40. I've shaded these values to make it
easier for you to check that there are
eight (38, 25, 37, 29, 36, 34, 22 and
36).
Now I can create a histogram of the results.
The vertical axis represents the frequency
of observations in each range:

Note that you can count the
frequencies as you create the
histogram. As you read the data
values put a cross in the appropriate
bin on the histogram.
Roll your mouse over the image
to see this.
