# Introduction

**History**

John Graunt, an English statistician, generally considered to be the founder of demography, the statistical study of human populations. Graunt along with William Petty developed early human statistical and census methods that later provided a framework for modern demography.

He put the data into tables and by interpreting that data, he compared the number of male and female births and deaths.

What is Data?

A set of facts or figures from which conclusions can be drawn. Data is plural of datum. Numerical data helps us to classify and analyse data for further action. Data is a collection of facts.

# Types of Data

Primary Data :-

# Secondary Data

Data which are not originally collected but obtained from published or unpublished sources are called secondary data.

Representing numerical data by using pictures or symbols is called a pictograph.

Pictographs are easy to read, express large number of data, but cannot express exact values.

This is the pictorial representation of games selected by people in a colony.

**Bar Graph :-**

A bar graph is another form of graphical representation of data, showing quantity of data according to the length of the bars. These bars can be either horizontal or vertical.

This bar graph represents ages of different animals. From this bar graph, we can get the age of animals.

**Comparison of Data :-**

We can represent two sets of data in a bar graph.

This bar graph shows average monthly expenditure in 2006 and also in 2007. This type of graph is called Double bar graph. We can compare two set of data using this double bar graph.