Collection of Data Class 11 NCERT: Complete Chapter 2 Guide

What Collection of Data Means in Statistics

In statistics, data means numerical or descriptive facts collected for a definite purpose. Collection of data is not just gathering random information; it is a systematic process guided by the objective of the enquiry. For example, if the objective is to study online learning habits of Class 11 students, useful data may include study hours, device used, internet access and preferred learning mode. Data about favourite sports may be interesting, but it is irrelevant unless connected to the objective. A good statistical enquiry begins by defining the problem, deciding the population, choosing primary or secondary sources, selecting a method, and checking whether the collected data is reliable, adequate and suitable.

Key Terms You Must Know

Primary data

Data collected first-hand by the investigator for a specific enquiry. It is original and usually more suitable, but it may take more time and money to collect.

Secondary data

Data already collected by someone else and used by the investigator for a new purpose. Examples include government reports, school records, newspapers and research publications.

Census method

A method in which information is collected from each and every unit of the population. It is detailed but often costly and time-consuming.

Sample method

A method in which information is collected from a representative part of the population. It is quicker and cheaper, but accuracy depends on how well the sample represents the population.

Questionnaire

A list of written questions prepared for collecting information from respondents, usually filled by the respondents themselves.

Schedule

A set of questions filled by an enumerator after asking respondents. It is useful when respondents may not understand the questions easily.

Steps in a Statistical Enquiry for Collecting Data

1. Define the objective clearly — State exactly what you want to study. A vague objective such as student habits is weak; a clear objective is average daily study time of Class 11 students in a school.
2. Identify the population and units — Decide whose information is required. The population may be all Class 11 students of a school, and each student is one statistical unit.
3. Choose primary or secondary data — Use primary data when fresh, specific information is needed. Use secondary data when reliable existing records already answer the question.
4. Select the collection method — For small and sensitive enquiries, direct personal investigation may work. For large areas, correspondents, mailed questionnaires, schedules or sampling may be better.
5. Check quality before conclusions — Data must be suitable for the objective, reliable in source, adequate in coverage and free from avoidable bias. Otherwise, later calculations may mislead.

Worked Examples: Choosing the Right Data Collection Method

• Example 1: A school wants to know how many Class 11 students use public transport. Step 1: Define the population: all Class 11 students of the school. Step 2: Since the population is small and available in one place, primary data is suitable. Step 3: A short questionnaire can ask mode of transport, distance from school and monthly travel cost. Step 4: If every Class 11 student is asked, it becomes a census within the school. Final answer: Use primary data through a short questionnaire, preferably census method if the number of students is manageable.
• Example 2: A student wants to study literacy rate changes in India over 30 years. Step 1: The objective requires historical national-level data. Step 2: Collecting primary data for past years is impossible. Step 3: Government census reports and official statistical publications are reliable secondary sources. Step 4: The student must check whether definitions of literacy stayed comparable across years. Final answer: Use secondary data from official published sources, with precautions about reliability and comparability.
• Example 3: An investigator wants to study household food expenditure in a city of 5 lakh households. Step 1: A census of all households would be very costly and slow. Step 2: A representative sample covering income groups and localities is more practical. Step 3: Because expenditure details may need explanation, schedules filled by trained enumerators can improve accuracy. Step 4: The sample should not include only one locality, otherwise it becomes biased. Final answer: Use primary data through sample method with schedules administered by enumerators.

Board Exam Tips and Common Traps

Do not write that primary data is always better than secondary data. Primary data is original, but secondary data may be more practical for historical, national or official information. Also, do not confuse questionnaire and schedule: in a questionnaire, respondents usually fill answers; in a schedule, an enumerator asks and records answers. In case-based questions, justify the method using words such as cost, time, accuracy, population size, literacy level of respondents and objective of enquiry. For secondary data, always mention the three checks: suitability, reliability and adequacy.

High-Value Points to Remember

• The objective of enquiry decides what data is relevant; extra facts are not automatically useful.
• Primary data can be collected by direct personal investigation, indirect oral investigation, information from correspondents, mailed questionnaires, schedules or telephonic interviews.
• Secondary data may be published or unpublished, but it must be examined before use.
• Census method is more detailed but expensive; sample method is economical but needs a representative sample.
• A good questionnaire uses simple, clear, non-leading questions and avoids double questions such as Do you study and revise daily?
• Bias can enter through poor sampling, unclear questions, dishonest responses or unsuitable secondary sources.

Practice Questions with Solutions

• Q: State whether the following should use primary or secondary data: A researcher wants to compare the population of Indian states in 2011. A: Step 1: The information relates to an already completed national population count. Step 2: It would be impractical and unnecessary to collect fresh data from all states. Step 3: Official census publications provide reliable published data. Final answer: Secondary data should be used, preferably from official census reports.
• Q: A teacher wants to know the preferred optional subject of 120 students in Class 11. Should she use census or sample method? A: Step 1: The population is only 120 students, which is small and easily reachable. Step 2: Asking every student will remove sampling error. Step 3: The cost and time involved are low because all students are in the same school. Final answer: Census method is suitable because the population is small and accessible.
• Q: Give two defects in this survey question: Do you agree that online classes are always better and more interesting than offline classes? A: Step 1: The phrase always better is leading because it pushes the respondent toward agreement. Step 2: The question combines two ideas, better and more interesting, so it is a double question. Step 3: A better version is: Which mode do you prefer for learning difficult topics: online, offline or blended? Give one reason. Final answer: The defects are leading wording and double-question structure.
• Q: An investigator uses a 10-year-old magazine article to estimate current unemployment among urban youth. Mention three precautions required before using it. A: Step 1: Check suitability: the article must match the objective, age group, area and definition of unemployment. Step 2: Check reliability: the original source, method of collection and credibility must be known. Step 3: Check adequacy: the data should be recent and broad enough for the present enquiry. Final answer: The investigator must check suitability, reliability and adequacy before using the secondary data.

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