Determine the number of Classes
•
If the number of classes is
–
too small
lack of detail
–
too large
some classes will be empty
•
Group all of the n data into K number of classes
•
General rule: K is the smallest whole number for
which 2
K
n
•
In Examples 2.2 n = 65
–
For K = 6, 2
6
= 64, < n
–
For K = 7, 2
7
= 128, > n
–
So use K = 7 classes
100
Number of Classes In General
Number of Classes
Size of Data Set
2
1
≤n<4
3
4≤n<8
4
8≤n<16
5
16≤n<32
6
32≤n<64
7
64≤n<128
8
128≤n<256
9
256≤n<528
10
528≤n<1056
Determine the Class Length
•
Find the
length
of each class as the largest
measurement minus the smallest divided by
the number of classes found earlier (K)
•
For Example 2.2, (2910)/7 = 2.7143
–
Because payments measured in days, round to
three days
102
Form NonOverlapping Classes of
Equal Width
•
The classes start on the smallest value
–
This is the lower limit of the first class
•
The upper limit of the first class is smallest value +
class length
–
In the example, the first class starts at 10 days and goes up
to 13 days
•
The next class starts at this upper limit and goes up
by class length
•
And so on
103
Seven NonOverlapping Classes Payment
Time Example
Class 1
10 days and less than 13 days
Class 2
13 days and less than 16 days
Class 3
16 days and less than 19 days
Class 4
19 days and less than 22 days
Class 5
22 days and less than 25 days
Class 6
25 days and less than 28 days
Class 7
28 days and less than 31 days
Tally and Count the Number of
Measurements in Each Class
Class
First 4 Tally
Marks
All 65 Tally
Marks
Frequency
10 < 13

3
13 < 16



14
16 < 19






23
19 < 22
I



12
22 < 25



8
25 < 28

4
28 < 31

1
Histogram
•
Rectangles represent the classes
•
The base represents the class length
•
The height represents
–
the frequency in a frequency histogram, or
–
the relative frequency in a relative frequency
histogram
106