There is no hard and fast rule about how many class we choose; but as a working rule the number of classes should lie between 5 and 15. It should be noted that the number of classes will be large if we choose small size class intervals and it will be small if the size of class intervals is large.
As an illustration, suppose the range is 70, and we choose classes of width 2 each. We would require 70 ÷ 2 = 35 classes. However, the number of classes would be 14 if the width of each class was 5.
Size of Class Intervals : We may choose all classes of the same width or of different width. In the case of equal class intervals the size of the class interval is determined as soon as we have decided about the number of classes.
Suppose n is the number of classes and all classes are of width h, then n × h = R.
Knowing the range R and number of classes
n we can abtain h = R/n as the width of class interval. If the range is 70 and we choose 10 classes, the width is 7.
Choice of Class Limits : Suppose x is a continuous variable, such that it can take any value in a given range. In that case, it is possible to choose class limits which are not equal to any of the observed values, For example, height of individuals is a continuous variable, even though, in practice, one can measure height to the nearest of the unit value (in centimetres) as 165, 170, 169, 171 .........; or to the nearest of tenth place of decimals as 165.3, 170.4, 168.9, 170.8, ........ We may specify class intervals as 160.55 165.55,165.55 ....... so that none of the observed values of x is equal to any of the class limits.
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Yogita Ingle 4 years, 11 months ago
There is no hard and fast rule about how many class we choose; but as a working rule the number of classes should lie between 5 and 15. It should be noted that the number of classes will be large if we choose small size class intervals and it will be small if the size of class intervals is large.
As an illustration, suppose the range is 70, and we choose classes of width 2 each. We would require 70 ÷ 2 = 35 classes. However, the number of classes would be 14 if the width of each class was 5.
Size of Class Intervals : We may choose all classes of the same width or of different width. In the case of equal class intervals the size of the class interval is determined as soon as we have decided about the number of classes.
Suppose n is the number of classes and all classes are of width h, then n × h = R.
Knowing the range R and number of classes
n we can abtain h = R/n as the width of class interval. If the range is 70 and we choose 10 classes, the width is 7.
Choice of Class Limits : Suppose x is a continuous variable, such that it can take any value in a given range. In that case, it is possible to choose class limits which are not equal to any of the observed values, For example, height of individuals is a continuous variable, even though, in practice, one can measure height to the nearest of the unit value (in centimetres) as 165, 170, 169, 171 .........; or to the nearest of tenth place of decimals as 165.3, 170.4, 168.9, 170.8, ........ We may specify class intervals as 160.55 165.55,165.55 ....... so that none of the observed values of x is equal to any of the class limits.
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