Ch 5: Sampling and Generalizability

Availability Sampling

A nonprobability sampling method in which elements are selected on the basis of convenience.

Census

Research in which information is obtained through responses from or information about all available members of an entire population.

Cluster

A naturally occurring, mixed aggregate of elements of the population.

Disproportionate Stratified Sampling

A sampling method in which elements are selected from strata in different proportions from those that appear in the population.

Elements

The individual members of the population whose characteristics are to be measured.

Enumeration Units

Units that contain one or more elements and that are listed in a sampling frame.

Inferential Statistics

A mathematical tool for estimating how likely it is that a statistical result based on data from a random sample is representative of the population from which the sample is assumed to have been selected.

Multistage Cluster Sampling

A sampling method in which elements are selected in two or more stages, with the first stage being the random selection of naturally occurring clusters and the last stage being the random selection of elements within clusters.

Nonprobability Sampling Method

A sampling method in which the probability of selection of population elements is unknown.

Nonrespondents

People or other entities who do not participate in a study although they are selected for the sample.

Periodicity

A sequence of elements (in a list to be sampled) that varies in some regular, periodic pattern.

Population

The entire set of individuals or other entities to which study findings are to be generalized.

Population Parameter

The value of a statistic, such as a mean, computed using the data for the entire population; a sample statistic is an estimate of a population parameter.

Probability of Selection

The likelihood that an element will be selected from the population for inclusion in the sample. In a census of all elements of a population, the probability that a particular element will be selected is 1.0. If half the elements in the population are sampled by chance (say, by tossing a coin), the probability of selection for each element is one half, or .5. As the size of the sample as a proportion of the population decreases, so does the probability of selection.

Probability Sampling Method

A sampling method that relies on a random, or chance, selection method so that the probability of selection of population elements is known.

Proportionate Stratified Sampling

A sampling method in which elements are selected from strata in exact proportion to their representation in the population.

Purposive Sampling

A nonprobability sampling method in which elements are selected for a purpose, usually because of their unique position.

Quota Sampling

A nonprobability sampling method in which elements are selected to ensure that the sample represents certain characteristics in proportion to their prevalence in the population.

Random Digit Dialing

The random machine dialing of numbers within designated phone prefixes, which creates a random sample for phone surveys.

Random Number Table

A table containing lists of numbers that are ordered solely based on chance; it is used for drawing a random sample.

Random Sampling

A method of sampling that relies on a random, or chance, selection method so that every element of the sampling frame has a known probability of being selected.

Random Sampling Error (Chance Sampling Error)

Differences between the population and the sample that are due only to chance factors (random error), not to systematic sampling error. Random sampling error may or may not result in an unrepresentative sample. The magnitude of sampling error resulting from chance factors can be estimated statistically.

Replacement Sampling

A method of sampling in which sample elements are returned to the sampling frame after being selected, so they may be sampled again. Random samples may be selected with or without replacement.

Representative Sample

A sample that “looks like” the population from which it was selected in all respects that are potentially relevant to the study. The distribution of characteristics among the elements of a representative sample is the same as the distribution of those characteristics among the total population. In an unrepresentative sample, some characteristics are overrepresented
or underrepresented.

Sample

A subset of a population that is used to study the population as a whole.

Sample Statistic

The value of a statistic, such as a mean, computed from sample data.

Sampling Error

Any difference between the characteristics of a sample and the characteristics of a population; the larger the sampling error, the less representative the sample.

Sampling Frame

A list of all elements or other units containing the elements in a population.

Sampling Interval

The number of cases from one sampled case to another in a systematic random sample.

Sampling Units

Units listed at each stage of a multistage sampling design.

Simple Random Sampling

A method of sampling in which every sample element is selected only based on chance, through a random process.

Snowball Sampling

A nonprobability sampling method in which sample elements are selected as they are identified by successive informants or interviewees.

Stratified Random Sampling

A sampling method in which sample elements are selected separately from population strata that are identified in advance by the researcher.

Systematic Bias

Overrepresentation or underrepresentation of some population characteristics in a sample resulting from the method used to select the sample; a sample shaped by systematic sampling error is a biased sample.

Systematic Random Sampling

A method of sampling in which sample elements are selected from a list or from sequential files, with every nth element being selected after the first element is selected randomly within the first interval.

Target Population

A set of elements larger than or different from the population sampled and to which the researcher would like to generalize study findings.