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Population

The entire group of individuals or instances about whom we hope to learn.

Sample

A (representative) subset of a population, examined in hope of learning about the population.

Sample Survey

A study that asks questions of a sample drawn from some population in the hope of learning something about the entire population.

Bias

Any systematic failure of a sampling method to represent its population.

Randomization

The best defense against bias. Each individual is given a fair, random chance of selection.

Matching

Any attempt to force a sample to resemble specified attributes of the population.

Sample Size

The number of individuals in a sample. Determines how well the sample represents the population, not the fraction of the population sampled.

Census

A sample that consists of the entire population.

Population Parameter

A numerically valued attribute of a model for a population. We rarely expect to know the true value, but we do hope to estimate if from sampled data.

Statistic

Values calculated for sampled data. Those that correspond to, and thus estimate, a population parameter, are of particular interest.

Representative

A sample is said to be this if the statistics computed from it accurately reflect the corresponding parameters.

Simple Random Sample

Each set of n elements in the population has an equal chance of selection.

Sampling Frame

A list of individuals from whom the sample is drawn.

Sampling Variability

The natural tendency of randomly drawn samples to differ, one from another. Sometimes results in Sampling Error.

Stratified Random Sample

A sample design in which the population is divided into several subpopulations, or strata, and random samples are then drawn from each stratum.

Cluster Sample

A sampling design in which entire groups are chosen at random.

Multistage Sample

Sampling schemes that combine several sampling methods.

Systematic Sample

A sample drawn by selecting individuals systematically from a sampling frame.

Voluntary Response Bias

Bias introduced to a sample when individuals can choose on their own whether to participate in the sample.

Convenience Sample

Consists of the individuals who are conveniently available.

Undercoverage

A sampling scheme that biases the sample in a way that gives a part of the population less representation than it has in the population.

Nonresponse Bias

Bias introduced to a sample when a large fraction of those sampled fails to respond.

Response Bias

Anything in a survey design that influences responses falls under this.