| Accessible population | The population from which the researcher can realistically select subjects for a sample, and to which the researcher is entitled to generalize findings.
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| Cluster random sampling | The selection of groups of individuals, called clusters, rather than single individuals. All individuals in a cluster are included in the sample; the clusters are preferably selected randomly from the larger population of clusters.
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| Convenience sampling | A nonrandom sampling technique based on convenient access to study participants.
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| Ecological generalizibility | The degree to which results can be generalized to environments and conditions outside the research setting.
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| External validity | The degree to which results are generalizable, or applicable, to groups and environments outside the research setting.
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| Generalizing | See ecological generalizability; population generalizability.
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| Nonrandom sampling | The selection of a sample in which every member of the population does not have an equal chance of being selected.
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| Periodicity | A phenomenon that occurs when an unexpected pattern coincides accidentally with the sampling interval resulting in biased results.
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| Population | The group to which the researcher would like the results of a study to be generalizable; it includes all individuals with certain specified characteristics.
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| Population generalizability | The extent to which the results obtained from a sample are generalizable to a larger group.
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| Purposive sampling | A nonrandom sample selected because prior knowledge suggests it is representative, or because those selected have the needed information.
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| Random sampling | Methods designed to select a representative sample by using chance selection so that biases will not systematically alter the sample.
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| Random start | Using random selection as the starting point for systematic sampling.
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| Replication | Refers to conducting a study again; the second study may be a repetition of the original study, using different subjects, or may change specified aspects of the study.
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| Representative sample | A sample that is like the population in terms of relevant characteristics.
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| Sample | The group on which information is obtained.
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| Sampling | The process of selecting a number of individuals (a sample) from a population, preferably in such a way that the individuals are representative of the larger group from which they were selected.
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| Sampling interval | The distance in a list between individuals chosen when sampling systematically.
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| Sampling ratio | The proportion of individuals in the population that are selected for the sample in systematic sampling.
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| Simple random sample | see random sample.
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| Stratified random sampling | The process of selecting a sample in such a way that identified subgroups in the population are represented in the sample in the same proportion as they exist in the population.
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| Systematic sampling | A selection procedure in which all sample elements are determined after the selection of the first element, since each element on a selected list is separated from the first element by a multiple of the selection interval.
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| Table of random numbers | A table of numbers that provides one of the best means of random selection or random assignment.
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| Target population | The population to which the researcher, ideally, would like to generalize results.
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| Two-stage random sampling | A process in which clusters are first randomly selected and then individuals are selected from each cluster.
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