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

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

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

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

Enumeration units
Units that contain one or more elements and that are listed ina sampling frame.

Sampling Units
Units listed at each stage of a multistage sampling design.

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.

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.

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 elements of a representative sample is the same as the distibution of those characteristics among the total population. In an unrepresentative sample, some characteristics are overrepresented or underrepresented.

Census
Research in which information is obtained through the responses that all available members of an entire population give to questions.

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.

Nonprobability Sampling Method
Sampling method in which the probability of selection of population elements is unknown.

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 any particular element will be selected is 1.0. If half of the elements in the population are sampled on the basis of 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 porbability of selection.

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.

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

Systematic Bias
Overrepresentation of some population characteristics in a sampel due to the method used to select the sample. A sample shaped by systematic sampling error is a biased sample.

Simple Random Sampling
A method of sampling in which every sample element is selected only on the basis of chance, through a random process.

Random Number Table
A table containing lists of numbers that are ordered solely on the basis of chance; it is used for drawing a random sample.

Random Digit Dialing
The random dialing by a machine of numbers withing designated phone prefixes, which creates a random sample for phone surveys.

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

Systematic Random Sampling
A method of sampling in which sample elements are selected from a list or from sequential files.

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

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

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

Proportionate Stratified Sampling
Sampling method in which elements are selected from strata in exact proportion to their representation in the population.

Disproportionate Stratified Sampling
Sampling in whcih elements are selcted from strata in different proportions from those that appear in the population.

Cluster Sampling
Sampling in whcih elements are selected in two or more stages, with the first stage being the random selection of naturally occuring clusters and the last stage being the random selection of elements within clusters.

Cluster
A naturally occuring, mixed aggregate of elements of the population.

Availability Sampling
Sampling in which elements are selected on the basis of convenience.

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.

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

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

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.

Random Sampling Error (Chance Sampling Error)
Differences between the population and the sameple 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 due to chance factors can be estimated statistically.

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

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

