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(While compiling this book I have made use of royalty free information available on the net. However at many places I have put it in much simpler language to make the concepts easier to understand. Again examples are incorporated. Thus this work is not totally original yet a lot of value addition is gone into the basic input.)

1.4 Common Statistical Terminology

Frequency
Number of times the observation repeats is called as frequency of that observation. In our example frequency of observation 200 is 2 while frequency of other observations is 1 each.

Class
Class is a group of the observations whose value fall in the specified range e,g. If we make classes of marks obtained by students as 11-20, 21-30 etc. then number of the students scoring marks between 11 to 20 will be recorded in class 11-20 and so on.

Class Boundary. (Class Limits, Class Length)
The extreme values which defines which observation would included in a class are called as class boundaries.

Upper class boundary and Lower class boundary:
The uppermost and lowermost values of a class are called as upper class boundary and lower class boundary respectively. In above example consider the class 11-20. The lower class limit is 11 and upper class limit is 20. Student getting marks 11 to 20 (11 and 20 both included) However, if classes are 10-20, 20-30 then the lower limit is include in the same class while the upper limit is included in the next class. That is observation 10 will be included in class 10-20 while observation 20 will be included in the class
20-30

Class Interval.
Difference in Upper class boundary and Lower class boundary is called as class interval. In above case class interval is 20-10=10.

Class Mark:
Mid point of a class is known as class mark. It is average of Upper class boundary and Lower class boundary i.e 15 is class mark for the class 20-30 .

Class Frequency
Class frequency is the number of observations in the class.