Introduction

Every study in economics rests on evidence, and that evidence usually takes the form of numbers. The purpose of collecting data is precisely to provide such evidence, so that a problem can be understood clearly and a sound conclusion reached. When we read a statement such as "food-grain output rose to 132 million tonnes in 1978-79 from 108 million tonnes in 1970-71, fell back to 108 million tonnes in 1979-80, then climbed steadily to 252 million tonnes in 2015-16 and 272 million tonnes in 2016-17," it is the data behind those figures that let us see and measure the fluctuations being described.

A quantity like food-grain production does not stay fixed — it changes from year to year and from crop to crop. A characteristic that takes different values in this way is called a variable, and variables are commonly denoted by letters such as $X$, $Y$ or $Z$. Each individual value that a variable takes is called an observation. For instance, if the years are represented by the variable $X$ and the food-grain production (in million tonnes) by the variable $Y$, then a value like 272 million tonnes for the year 2016-17 is one observation of $Y$. The full set of such values of $X$ and $Y$ constitutes the data, and it is from these data that we obtain information about, say, how food-grain output has behaved over time.

Table 2.1 below shows exactly this — the food-grain production data for India across these years.

Seen this way, data are a tool — they help us understand a problem by supplying information about it. But this immediately raises two questions: where do data come from, and how are they collected? These are the questions the chapter sets out to answer.

Studying this chapter should enable you to:

• understand the meaning and purpose of data collection;
• distinguish between primary and secondary sources of data;
• know the modes of collecting data — personal interviews, mailed questionnaires and telephone interviews — and how to prepare a good questionnaire;
• distinguish between a Census (complete enumeration) and a Sample Survey;
• become familiar with the techniques of sampling (random and non-random) and with sampling and non-sampling errors; and
• know some important sources of secondary data, such as the Census of India and the National Sample Survey.

In short, the chapter moves from what data are, through how they are gathered and from whom, to which agencies make reliable data available — with the constant reminder that the choice of source and method of collection must always fit the objective of the study.