Data may be obtained either from primary source or the secondary source. A primary source is one that itself collects the data, a secondary data source is one that makes available data which were collected from some other agency. Depending on the source, statistical data are classified under two categories :-
(i) Primary data and (ii) Secondary data

Primary data are original and first hand information data which are not originality collected but rather obtained from published or unpublished sources are known as secondary data. For example any government publication is first hand (Primary data) for government and second hand (secondary data) for a research worker. Method of collecting primary data:-

Primary data may be obtained by applying any of the following method:-

• Direct personal interviews
• Indirect personal interviews
• Information form correspondence
• Mailed questionnaire
• Questionnaire to be filled by enumerators.

Direct Personal interviews: –
According to this method data are collected by the investigator personally from person who is subject to enquiry. Researcher interviews personally to everyone who is in position of supply information he requires. One can use this method when areas of enquiry is limited or when a maximum degree of accuracy is needed.

Indirect Personal Interviews: –
Direct personal enquiry cannot be used in the case of the refusal and reluctance of the person who are to be interviewed. Then an indirect personal enquiry can be conducted to get necessary information from an indirect source. Person who has the knowledge of relevant material or event are interviewed and asked questions for collecting the data. Information from correspondents: In this method local agents or correspondents are appointed in different part of the investigation area. These agents regularly supply the information to the central office or investigator. They collect the information according to their own judgment and own methods.

Mailed Questionnaire:
In this method a list of questions relating to inquiry which is known as schedule or questionnaire is prepared by the researcher. This list of questions provides a space for each answer. Schedules are sent to informants by post, with request to answer and return it within a specified time.

Questionnaire to be filled by enumerators:
Mailed questionnaire method posses a number of difficulties in collection of data. Generally these filled questionnaire received are incomplete, Inadequate and unrepresentative. The second alternative approach is to send trained investigator or enumerators to respondent with standardized questionnaire which are to be filled by the investigator. The investigator helps the informants in recording their answers. It is an expensive method as compared to other method of primary collection of data as the enumerators are required to be paid.

Collection of Secondary Data:
Secondary data are those which are collected by some other agency and are used for further studies. It is not necessary to conduct special surveys and investigations. We can obtain the required statistical information from other institution or reports which are already published by them as part of their routine work. Secondary data may be either published either published or unpublished.

Published Data :
Government Publication, Publication of international organisation, Semi Official Publication, Reports of Committees and Commissions, Private Publications such as Journal and news paper, research institutions, Professional trade bodies, research articles, reviews and reports.

Unpublished Data :
Research institutions, trade associations, universities, labour bureaus and research scholars do collect date but they normally do not publish it. Researcher can use such data for the purpose of his investigation.