SM1.2 Sampling Methods

Population

The entire group of individuals or items about which information is desired

Census and potential disadvantages

Definition: A survey collecting data from every member of the population, usually conducted systematically and periodically (e.g., national census every 10 years) .

Disadvantage: Extremely expensive and time‑consuming compared to sampling alternatives.

Sample Survey and potential disadvantage

Definition: Data collected from a selected subset (sample) of the population, used to make inferences about the whole

Disadvantage: Susceptible to sampling error and may not accurately represent the population if the sample is poorly chosen.

Sampling Frame and potential disadvantage

Definition: A list or database from which you draw your sample (e.g., voters register, phone directory) .

Disadvantage: Missing or outdated entries can bias the sample (frame error)

Simple Random Sampling and potential disadvantage

Definition: Choosing a sample such that every member of the sampling frame has an equal chance of selection .

Disadvantage: Impractical when the population is huge or the frame is incomplete; can miss small subgroup

Systematic Sampling and potential disadvantage

Definition: From an ordered list, select every nth individual (after a random start) .

Disadvantage: Risk of periodicity bias if there's a pattern in the list (e.g., selecting every 7th day misses weekend trends).

Stratified Sampling and potential disadvantages

Definition: Divide population into subgroups (strata), then random‑sample within each; strata are mutually exclusive and exhaustive .

Disadvantage: Needs detailed knowledge and resources to classify everyone; complex to implement

Quota Sampling and potential disadvantages

Definition: Split the population into subgroups, then use non‑random methods (like convenience or judgement) to fill quotas for each subgroup .

Disadvantage: Non‑random selection introduces biases that are hard to detect or adjust for.