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
A systematic failure of a sampling method to represent its population is bias. It is almost impossible to recover from bias, so effort to avoid it are well spent. Common errors include
- Relying on voluntary response.
- Under-coverage of the population.
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Randomization
the best defense against bias, in which 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
population count.
Population parameter
a numerically valued attribute of a model for a population.
Statistic, sample statistic
values calculated for sampled data. Those that correspond to, and thus estimate, a population parameter, are of particular interest. For example, the mean income of all employed people in a representative sample can provide a good estimate of the correspo
representative
this kind of sample accurately reproduces the characteristics of the population a researcher is studying
Simple random sample (SRS)
process of taking a sample n in which each set of n elements in the population has an equal chance of selection.
Sampling frame
the individuals or clusters of individuals who might actually be selected for inclusion in the sample
sampling variability
the natural tendency of randomly drawn samples to differ, one from another; the natural result of random sampling.
Stratified random sample
A sampling 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. Usually selected as a matter of convenience, practicality or cost. each cluster should be diverse in character and representative of the population, so all the groups should be similar to each
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
sample consisting 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. Those who do respond are likely to not represent the entire sample. Voluntary response bias is a one form. For example, those who are at work during the day won't respond
Response bias
Anything in a survey design that influence response. Typically arises from the wording of questions, which may suggest a favored response. Voters, for example, are more likely to express support of "the president" than support of the particular person hol