(i) Observations:- 

Data obtains by the observer from the given problem is called observations. 

(ii) Raw data:- 

Data collected in its original form is called Raw Data. 

(iii) Fequency of an observation:- 

Number of times a certain value occurs is termed as frequency. 

(iv) Frequency distribution:- 

Organisation of Raw data in tabular form is called Frequency Distribution. 

(v) Discrete frequency distribution:- 

Frequency Distribution in which Raw Data is ungrouped is called Discrete frequency distribution. 

(vi) Grouped frequency distribution:- 

A frequency Distribution in which Raw data is grouped in specific class.

(vii) Class - interval:- 

Class interval is a group under which large number of data is grouped to analyze its Range and Distribution. 

(viii) Class - size:- 

Class – size is defined as the difference between upper and lower boundaries of any class. 

(ix) Class limits:- 

Class limits works as separation of one class in a grouped frequency distribution from another. 

(x) True class limits:- 

The Exact class limits of frequency distribution are called True class limits.