Revision summary
Sampling selects units so researchers can speak carefully about a population. Probability samples give known chances of selection and estimable error. Stratified designs aid comparison; cluster designs save cost and inflate error. Non-probability samples help exploratory and hidden-population work. Convenience and quota designs are often misused as if they were random. Match the sampling form to the claim: national rates versus theoretical cases.
Model answer
Introduction
Sampling in social research is the selection of a subset of persons, households, organisations or events from a defined population so that we can say something disciplined about the whole. A census is rarely possible. The quality of a survey on caste, voting or nutrition stands or falls with how the sample was drawn. Different forms of sampling trade representativeness against cost, access and the need for depth.
Body
Probability sampling
In probability designs every unit has a known, non-zero chance of selection, which allows sampling error to be estimated.
Simple random sampling gives each unit equal chance. It is unbiased in theory and clumsy in practice when no complete list exists, as in an unlisted slum. Systematic sampling takes every kth unit from a list; it is convenient but dangerous if the list has a hidden period. Stratified sampling divides the population into strata—region, caste group, rural/urban—and samples within each. It improves precision for comparisons and needs good stratum information. Cluster and multistage sampling, used by NSSO and NFHS, select villages or blocks first, then households. They cut travel costs and raise design effects: people in a cluster are alike, so effective sample size shrinks. Advantages of probability samples are generalisability and honest margins of error. Disadvantages are cost, non-response, outdated frames, and ethical or political difficulty of listing stigmatised populations.
Non-probability sampling
Convenience samples of students or online volunteers are cheap and weak for inference. Purposive samples choose theoretically important cases, essential in ethnography and case comparison. Quota samples mimic margins of sex or age without random selection; they look representative and hide interviewer bias. Snowball samples follow networks and are often the only way to reach hidden populations—drug users, undocumented migrants, queer networks—but they over-represent the well connected.
No single form is best. A national poverty estimate needs a probability design. A study of a new sect may need snowballing. Mixed designs are common. The sociologist’s duty is to match form to claim: do not generalise a campus convenience sample to “Indian youth.”
Flow diagram
flowchart TD POP[Population] --> PR[Probability] POP --> NP[Non-probability] PR --> SRS[Simple systematic stratified] PR --> CL[Cluster multistage] NP --> PU[Purposive quota] NP --> SN[Snowball convenience]
Conclusion
Sampling is the bridge from studied cases to a population. Probability forms support statistical inference at a price; non-probability forms support access and theory at the price of unknown bias. Advantages and disadvantages are not abstract: they decide whether a sociological statement is a finding or an anecdote.
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