Data may be collected by two methods :

(1) Census method

(2) Sample method

1. Census Method : Under this method, each and every unit of the universe under survey is studied. For example, if there are 32 students in the class, than the marks of all the 32 students of the class will be collected.

Advantages : This method has following advantages:

1. Results will be absolutely correct, accurate and reliable.

2. There is less possibility of partiality in collecting the data.

3. Extensive knowledge of the problem is possible.

4. Only this method useful, when the units of the universe have different characteristics.

Disadvantages : 1. This method requires a lot of time, energy and money.

2. This method cannot be applied in some cases.

3. It is an unscientific method.

4. It can be used only in a small universe.

2. Sample Method : Under this method,

data of each and every unit of the universe is not collected. But data of some representative units of the universe are collected and results may be drawn on the basis of these data for the entire universe.

Advantages : 1. Sample method is easy and scientific.

2. It requires less time and money.

3. This method may be used in those circumstances in which census method may not be used successfully.

Disadvantages : 1. There is possibility of misusing the data.

2. If wrong samples are taken, the analysis will be wrong.

3. This method is applicable only in that universe where various units are similar to each other.

4. The results derived with the help of this method may not be perfectly correct.

The following are the methods of primary data collection which are in common use :

1. Direct Personal Interview : According to this method, data are collected by the investigator personally from persons who are the subject to enquiry. He interviews personally every one who is in a position to supply information he requires. We can use this method of collection of data when area of enquiry is limited or when a maximum degree of accuracy is needed. One can think of personal enquiry in a case of collection of information regarding family budget and living conditions in a group area. The investigator must be skilled, tactful, accurate, pleasing and should not be biased.

2. Indirect Personal Interview : Direct personal enquiry cannot be used in the case of the refusal and reluctance of the persons who are to be interviewed. Then an indirect personal enquiry can be conducted to get a necessary information from an indirect source. Persons who have the knowledge of relevant material are interviewed and asked questions for collecting the data.

3. In this method, local residents or correspondent are appointed in the different parts of the investigation area. These agents regularly supply the information to the central office or investigator. They collect the information according to their own judgements. Radio and newspaper agencies generally obtain information about strikes, thefts, accidents etc. by this method, It is adopted by Government departments to get estimates of agricultural crops and the wholesale price index number. It is suitable when the information is to be obtained from a wide area and where a high degree of accuracy is not required.

4. Mailed questionnaires : A list of questions relating to inquiry, which is called schedule or questionnaire, is prepared. The list of questions provides a space for each answer. Schedules are sent to informants by post, with a request to answer and return it within a specified time. Such schedules generally have prepaid postage stamp affixed to them. If necessary, an assurance is given that the answers will be kept confidential.

5. Questionnaire to be filled by enumerators : Mailed questions method poses a number of difficulties in collection of data. Generally questionnaires received are incomplete, inadequate and unrepresentatives. The second alternative approach is to send trained investigators or enumerators to informants with standardized questionnaire which are to be filled in by the investigators. The investigator help the informations in recording their answers. The investigators should behonest, tactful and painstaking. This is the most common method used by research organisations.

Secondary data may be either (a) Published or (b) Unpublished.

Published Sources :

(i) Government Publications : Different ministers and departments of Central and State

Governments publish regularly current information along with statistical data on a number of subjects. This information is quite reliable for related studies. The examples of such publications are : Annual Survey of Industries, Labour Gazette, Agriculture Statistics of India.

(ii) Publication of International Organization : We can obtain valuable international statistics from official publication of different international organizations, like, The United Nations Organisation (U.N.O.) International Labour Organisation (I.L.O.) International Monetary Fund (I.M.F.) etc.

(iii) Semi-official Publications : Local bodies such as Municipal Corporations, District Boards etc. publish periodical reports which provide factual information about health, sanitation, births, deaths etc.

(iv) Report of committees and commissions : Various committees and commissions are appointed by the Central and State Governments for some special studies and recommendations. The reports of such committees and commissions contain valuable data. Some of the report are : Report of National Agricultural Commission, Report of the Tariff Commission, the Patel Committee Report etc. Unpublished Sources : Research institutions, trade associations, universities, labour bureau, research workers and scholars do collect data but they normally do not publish it. Apart from the above sources we can get the information from records and files of Government and private offices.

In statistics universe (population) refers to an aggregate of all items about which we want to obtain information. Suppose there are 1500 studetns in a particular school. If an investigation relates to all the 1500 students, then 1500 would be taken as universe or population.

On the other hand, if we take out only 150 for investigation, then 150 students will be taken as sample. Sample is only the part of the population or the universe. This part must represent the characteristics of universe. Population and sample have been projected in the figures. Figure (a) represents the universe and figure (b) represents the sample.

Difference between census method and sample methods:

In order to draw correct conclusion, it is necessary that the sample should be reliable. Unrealiable sample gives incorrect data and the conclusions drawn may not show the features of the population. Reliability of sample depends upon a number of factors.

1. Size : Choice of an appropriate sampling method increases the reliability of data.

2. Sampling errors : Sampling errors reduce the reliability of sample. A sampling error arises due to drawing of inference about the population on the basis of a few observations. There are two types of sampling errors : 1. Biased error, 2. Unbiased error.

3. Non sampling errors : They are more serious than sampling errors because a sampling errors because a sampling error can be minimised by taking a larger sample. It is difficult to minimise non-sampling error, even by taking a large sample. Some of the non-sampling errors are :

(a) Errors in data acquisition

(b) Non response errors

(c) Sampling bias.