Statistics for SSC-CGL Tier-II (Paper-III) – Part-4

Categories of data

Any statistical data can be classified under two categories depending upon the sources utilized.

These  categories are,

1. Primary data

2.  Secondary data

Primary data:

Primary data is the one, which is collected by the investigator himself for the purpose of a specific inquiry or study.

Such data is original in character and is generated by survey conducted by individuals or research institution or any organisation.


If a researcher is interested to know the impact of noon-meal scheme for the school children, he has to undertake a survey and collect data on the opinion of parents and children by asking relevant questions. Such a data collected for the purpose is called primary data.

The primary data can be collected by the following five methods.

1. Direct personal interviews.

2. Indirect Oral interviews.

3. Information from correspondents.

4. Mailed questionnaire method.

5. Schedules sent through enumerators.

1. Direct personal interviews:

The persons from whom informations are collected are known as informants.  The investigator personally meets them and asks questions to gather the necessary informations.  It is the suitable method for intensive rather than extensive field surveys.  It suits best for intensive study of the limited field.


1. People willingly supply informations because they are approached personally.  Hence, more response noticed in this method than in any other method.

2. The collected informations are likely to be uniform and accurate.  The investigator is there to clear the doubts of the informants.

3. Supplementary informations on informant’ s personal aspects can be noted.  Informations on character and environment may help later to interpret some of the results.

4. Answers for questions about which the informant is likely to be sensitive can be gathered by this method.

5. The wordings in one or more questions can be altered to suit any informant.  Explanations may be given in other languages also.  Inconvenience and misinterpretations are thereby avoided.


1. It is very costly and time consuming.

2. It is very difficult, when the number of persons to be interviewed is large and the persons are spread over a wide area.

3. Personal prejudice and bias are greater under this method.

2. Indirect Oral Interviews:

            Under this method the investigator contacts witnesses or neighbours or friends or some other third parties who are capable of supplying the necessary information.  This method is preferred if the required information is on addiction or cause of fire or theft or murder etc., If a fire has broken out a certain place, the persons living in neighbourhood and witnesses are likely to give information on the cause of fire.  In some cases, police interrogated third parties who are supposed to have knowledge of a theft or a murder and get some clues.  Enquiry committees appointed by governments generally adopt this method and get people’ s views and all possible details of facts relating to the enquiry.  This method is suitable whenever direct sources do not exists or cannot be relied upon or would be unwilling to part with the information.

The validity of the results depends upon a few factors, such as the nature of the person whose evidence is being recorded, the ability of the interviewer to draw out information from the third parties by means of appropriate questions and cross examinations, and the number of persons interviewed.   For the success of this method one person or one group alone should not be relied upon.


3. Information from correspondents:

The investigator appoints local agents or correspondents in different places and compiles the information sent by them.

Informations to Newspapers and some departments of Government come by this method.  The advantage of this method is that it is cheap and appropriate for extensive investigations.  But it may not ensure accurate results because the correspondents are likely to be negligent, prejudiced and biased.  This method is adopted in those cases where information is to be collected periodically from a wide area for a long time.

4.  Mailed questionnaire method:

Under this method a list of questions is prepared and is sent to all the informants by post.  The list of questions is technically called questionnaire. A covering letter accompanying the questionnaire explains the purpose of the investigation and the importance of correct information and request the informants to fill in the blank spaces provided and to return the form within a specified time.  This method is appropriate in those cases where the informants are literates and are spread over a wide area.


1. It is relatively cheap.

2. It is preferable when the informants are spread over the wide area.


1. The greatest limitation is that the informants should be literates who are able to understand and reply the questions.

2. It is possible that some of the persons who receive the questionnaires do not return them.

3. It is difficult to verify the correctness of the information furnished by the respondents.

With the view of minimizing non-respondents and collecting correct information, the questionnaire should be carefully drafted.  There is no hard and fast rule.  But the following general principles may be helpful in framing the questionnaire.  A covering letter and a self addressed and stamped envelope should accompany the questionnaire.  The covering letter should politely point out the purpose of the survey and privilege of the respondent who is one among the few associated with the investigation.  It should assure that the informations would be kept confidential and would never be misused.  It may promise a copy of the findings or free gifts or concessions etc.,

Qualities of a good questionnaire:

1. Number of questions should be minimum.

2. Questions should be in logical orders, moving from easy to more difficult questions.

3. Questions should be short and simple.  Technical terms and vague expressions capable of different interpretations should be avoided.

4. Questions fetching YES or NO answers are preferable. There may be some multiple choice questions requiring lengthy answers are to be avoided.

5. Personal questions and questions which require memory power and calculations should also be avoided.

6. Question should enable cross check. Deliberate or unconscious mistakes can be detected to an extent.

7. Questions should be carefully framed so as to cover the entire scope of the survey.

8. The wording of the questions should be proper without hurting the feelings or arousing resentment.

9. As far as possible confidential information should not be sought.

10. Physical appearance should be attractive, sufficient space should be provided for answering each questions.

5.  Schedules sent through Enumerators:

Under this method enumerators or interviewers take the schedules, meet the informants and filling their replies.  Often distinction is made between the schedule and a questionnaire.  A schedule is filled by the interviewers in a face-to-face situation with the informant.  A questionnaire is filled by the informant which he receives and returns by post.  It is suitable for extensive surveys.


1. It can be adopted even if the informants are illiterates.

2. Answers for questions of personal and pecuniary nature can be collected.

3. Non-response is minimum as enumerators go personally and contact the informants.

4. The information collected are reliable. The enumerators can be properly trained for the same.

5. It is most popular methods.


1. It is the costliest method.

2. Extensive training is to be given to the enumerators for collecting correct and uniform information.

3. Interviewing requires experience. Unskilled investigators are likely to fail in their work.

Before the actual survey, a pilot survey is conducted.  The questionnaire/Schedule is pre-tested in a pilot survey.  A few among the people from whom actual information is needed are asked to reply.  If they misunderstand a question or find it difficult to answer or do not like its wordings etc., it is to be altered.  Further it is to be ensured that every questions fetches the desired answer.

Merits and Demerits of primary data:

1. The collection of data by the method of personal survey is possible only if the area covered by the investigator is small. Collection of data by sending the enumerator is bound to be expensive. Care should be taken twice that the enumerator record correct information provided by the informants.

2. Collection of primary data by framing a schedules or distributing and collecting questionnaires by post is less expensive and can be completed in shorter time.

3. Suppose the questions are embarrassing or of complicated nature or the questions probe into personnel affairs of individuals, then the schedules may not be filled with accurate and correct information and hence this method is unsuitable.

4. The information collected for primary data is mere a reliable than those collected from the secondary data.

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