011. Class Intervals and Midpoints

The Class interval is the range of values found within a class. It is determined by subtracting the lower (or upper) boundary of one class from the lower (or upper) boundary of the next class. The interval of the first class in Table 2.3.is thus 60 – 50 = 10.

It is desirable to make all class Intervals of Equal size in a frequency distribution. However, equal class sizes may not always be possible. It may necessary to use Open – ended intervals, which do not cite a lower boundary for the first class or an upper boundary for the last class. The last class in Table 2.4 might be stated as “600 and up”.

The class interval can be determined by the following expression:

. (2.3)

For the example 2.2 . Since 8.7 is an awkward number, the results can be slightly adjusted up or down to facilitate construction of the frequency table. For convenience, the interval of 10 was selected in forming Table 2.3.

It is often necessary to determine a Class midpoint. This is done by calculating the average of the boundaries of a class by adding the upper and lower boundaries and dividing by 2. thus, the midpoint for the first class in Table 2.5 is (50+59)/2 = 54.5. The remaining midpoints are also shown.

Table 2.5 – Frequency Distribution and Midpoints for Passengers

Class	Frequency	Midpoint
50 to 59	3	54.5
60 to 69	7	64.5
70 to 79	18	74.5
80 to 89	12	84.5
90 to 99	8	94.5
100 to 109	2	104.5

< Предыдущая		Следующая >