A research problem is identified, and the need to receive the necessary information is determined. The next step is to look for the sources of data to fulfill the research objectives. Generally, researchers plunge into data collection by doing a survey. However, it is desirable that the secondary source of data are tapped first to get the required information. The primary data is then collected to meet the gaps in the desired information. Secondary data should be tapped first before collecting the primary data.

Broadly, data are put into two categories ( a ) primary data and ( b )  secondary data . Primary data means to generate the data first hand with respect to a problem at hand by doing surveys, using panels, doing observations or experimentation. Secondary data, on the other hand,is data already collected beforehand by someone and which has relevance for the study the researcher has undertaken.

We can illustrate the distinction between the primary data and secondary data by taking an example. Suppose we conduct a study the ‘ effectiveness of Fair and Lovely TV commercials ‘ in 2001. Later in 2003, again there was a need to conduct the same study. We can use some relevant data from 2001 study, which is the use of secondary data. The additional data can be collected by survey, which we call the primary data. Thus the primary data of the 2001 one study becomes the secondary data for 2003 study.

A research problem can be tackled partially at least by using the secondary data lying scattered across several sources. After exhausting these existing sources of secondary data, we shall resort to primary data collection. But the use of secondary data must be intelligent. We should be aware of the metodology by which the data are collected. We should be in a position to evaluate the secondary data. For this we must be familiar with the techniques of collecting primary data. If the available secondary data is not usable, we should go in for primary data 100 per cent. Secondary data are valuable if the present problem has relationship with earlier problems.