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bell curve
Smoothed histogram or bar graph describing the expected frequency for each value of a variable. The name comes from the fact that such a distribution often has the shape of a bell.

census
A kind of survey that involves a complete enumeration of the entire population of interest.

cluster or area random sampling
A sampling method that involves dividing the population into groups called clusters, randomly selecting clusters, and then sampling each element in the selected clusters. This method is useful when sampling a population that is spread across a wide geographic area.

concept maps
Two dimensional graphs of a group's ideas where ideas that are more similar are located closer together and those judged less similar are more distant. Concept maps are often used by a group to develop a conceptual framework for a research project.

expert sampling
A sample of people with known or demonstrable experience and expertise in some area.

external validity
The degree to which the conclusions in your study would hold for other persons in other places and at other times.

generalizability
The degree to which study conclusions are valid for members of the population not included in the study sample.

gradient of similarity
The dimension along which your study context can be related to other potential contexts to which you might wish to generalize. Contexts that are closer to yours along the gradient of similarity of place, time, people, and so on can be generalized to with more confidence than ones that are further away.

heterogeneity sampling
Sampling for diversity or variety.

modal instance sampling
Sampling for the most typical case.

multistage sampling
The combining of several sampling techniques to create a more efficient or effective sample than the use of any one sampling type can achieve on its own.

nonprobability sampling
Sampling that does not involve random selection.

nonproportional quota sampling
A sampling method in which you sample until you achieve a specific number of sampled units for each subgroup of a population, where the proportions in each group are not the same.

population
The group you want to generalize to and the group you sample from in a study.

population parameter
The mean or average you would obtain if you were able to sample the entire population.

probability sampling
Method of sampling that utilizes some form of random selection.

proportional quota sampling
A sampling method in which you sample until you achieve a specific number of sampled units for each subgroup of a population, where the proportions in each group are the same.

proximal similarity model
A model for generalizing from your study to another context based upon the degree to which the other context is similar to your study context.

quota sampling
Any sampling method in which you sample until you achieve a specific number of sampled units for each subgroup of a population.

random selection
Process or procedure that assures that the different units in your population are selected by chance.

response
A specific measurement value that a sampling unit supplies.

sample
The actual units you select to participate in your study.

sampling distribution
The theoretical distribution of an infinite number of samples of the population of interest in your study.

sampling error
The error in measurement associated with sampling.

sampling frame
The list from which you draw your sample. In some cases, there is no list; you draw your sample based upon an explicit rule. For instance, when doing quota sampling of passersby at the local mall, you do not have a list per se, and the sampling frame consists of both the population of people who pass by within the time frame of your study and the rule(s) you use to decide whom to select.

sampling model
A model for generalizing in which you identify your population, draw a fair sample, conduct your research, and finally, generalize your results to other population groups.

simple random sampling
A method of sampling that involves drawing a sample from a population so that every possible sample has an equal probability of being selected.

snowball sampling
A sampling method in which you sample participants based upon referral from prior participants.

standard deviation
The spread or variability of the scores around their average in a single sample. The standard deviation, often abbreviated SD, is mathematically the square root of the variance. The standard deviation and variance both measure dispersion, but because the standard deviation is measured in the same units as the original measure and the variance is measured in squared units, the standard deviation is usually more directly interpretable and meaningful.

standard error
The spread of the averages around the average of averages in a sampling distribution.

statistics
The process of estimating various features from data, often using probability theory.

stratified random sampling
A method of sampling that involves dividing your population into homogeneous subgroups and then taking a simple random sample in each subgroup.

systematic random sampling
A sampling method in which you determine randomly where you want to start selecting in the sampling frame and then follow a rule to select every xth element the sampling frame list (where the ordering of the list is assumed to be random).

validity
The best available approximation of the truth of a given proposition, inference, or conclusion.

