Collection of Data — Class 11 Economics

After knowing why data matter, the next step is to collect them, since data help understand a problem by providing evidence. A quantity that varies, like food-grain output across years, is a variable (denoted X, Y or Z), and each value is an observation. Data come from two sources: primary data are gathered first-hand through an enquiry, while secondary data have already been collected and processed by another agency and reused from reports, books, newspapers or websites; the same figures are primary to the body that first compiles them and secondary to all who borrow them later. A survey gathers information through a questionnaire whose questions should be short, clear and unambiguous, avoid double negatives and leading phrasing, and move from general to specific. Data can be collected by personal interview, mailed questionnaire or telephone interview, differing in cost, response rate and reach; a pilot survey pre-tests the questionnaire first. A survey may cover every unit (a census or complete enumeration, done in India every ten years) or only a representative sample, which is cheaper. Samples are chosen by random methods, where every unit has an equal chance, or non-random methods relying on judgement. Errors are of two kinds: sampling errors, the gap between a sample estimate and the true value, which shrink with larger samples, and non-sampling errors from bias, non-response or faulty recording. The Census of India and the National Sample Survey are key agencies.