5 min readConcept sheetUsually a 2- or 5-mark answer

Primary and secondary data

Primary data are collected first-hand for your own study. Secondary data were collected earlier by someone else and are reused, such as NFHS reports.

PrimaryOriginal, first-hand; e.g. your own diet survey

SecondaryAlready collected by others; e.g. NFHS, Census, hospital records

Check secondary data forReliability, suitability, adequacy (Kothari)

Primary vs secondary data

Point	Primary data	Secondary data
Collected by	The researcher, for this study	Someone else, for another purpose
Nature	Original, first-hand	Already available, second-hand
Cost and time	High	Low
Fit to the problem	Fits the objectives exactly	Definitions, units or area may not match
Accuracy	Known, because you control the method	Depends on the original collector
Methods or sources	Observation, interview, questionnaire, schedule, measurement	Published reports, records, journals, websites
Nutrition example	24-hour diet recall of 100 students	Anaemia figures from NFHS-5

Methods and sources

Collecting primary dataObservation, interview, questionnaire, schedule and direct measurement. In nutrition this is the ABCD of assessment: anthropometric, biochemical, clinical and dietary data.

Sources of secondary dataPublished: Census, NFHS, NNMB reports, ICMR-NIN dietary guidelines, Indian Food Composition Tables (IFCT 2017), WHO reports, journals. Unpublished: hospital diet records, school health registers, theses.

Most good studies use both: secondary data to plan and compare, primary data to answer your own question.

Before using secondary data, check (Kothari)

Reliability: who collected it, by what method, when (is it recent enough?), and was there any bias?
Suitability: do the definitions, units and area match your study? (NFHS counts girls aged 15-19 as adolescents.)
Adequacy: does it cover enough of your area, period and group?

In nutrition research

A dietitian uses secondary data (NFHS-5 anaemia rates, IFCT 2017 nutrient values) to plan, and collects primary data (weight, Hb, 24-hour recall) to assess her own group. A study of anaemia in college girls can compare its own figures with NFHS-5.

Exam angle

2 marks: “Define primary data.” Primary data are original data collected first-hand by the researcher for the specific purpose of the study, e.g. through a questionnaire or measurement.

5 marks: “Differentiate between primary and secondary data.” Compare who collects, originality, cost, time, fit to the problem and accuracy, with one nutrition example each; add Kothari's three checks for secondary data.

10 marks: “What are primary and secondary data? Explain their sources and the precautions in using secondary data.” Definitions, methods of primary data, published and unpublished secondary sources, a comparison table, and reliability, suitability and adequacy.

Remember it

Primary = Personally collected. Secondary = Someone else collected it.