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Class 11 Collection of Data: cheat sheet
CBSE Social Science. Primary and secondary data, questionnaires and the three ways of interviewing, census and sample surveys, random sampling, and the errors that creep in.
Key ideas
- Primary data are collected first-hand by the investigator; secondary data come from sources that collected them earlier, such as reports.
- A good questionnaire is short and clear, moves from general to specific, and avoids double negatives, leading questions and suggested alternatives.
- Personal interviews give the highest response but cost most; mailed questionnaires are cheapest and reach remote areas; telephone interviews are quick but miss people without phones.
- A pilot survey tries the questionnaire on a small group first to find weak questions and judge cost and time.
- A census covers every unit; a sample survey studies a representative part and is cheaper, quicker and allows more detail.
- In random sampling every unit has an equal chance of selection, as in a lottery.
- Sampling error falls as the sample grows; non-sampling errors (wrong recording, non-response, bias) do not, even in a census.
Remember
| Sampling error | sample average − population average (as a positive gap) |
| Chance of selection | 1 in (population ÷ sample size) |
| Response rate | returned ÷ sent × 100 |
| Sample size | per cent × population ÷ 100 |
| Saving from sampling | households × cost each × (100 − sample %) ÷ 100 |
| Census of India | every ten years, regularly since 1881 |
Watch out
- Thinking a census has no errors: non-sampling errors can occur in it too.
- Calling a “Don’t you think…?” question neutral: it pushes the answer.
- Mixing up the population (all units) with the sample (the part studied).
Worked example
The incomes (₹) of a population of five farmers are 1,300, 750, 600, 450, 1,200. Find the population average, the average of the sample 1,300 and 450, and the sampling error.
Working: 4,300 ÷ 5; (1,300 + 450) ÷ 2
Answer: ₹860, ₹875, ₹15
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