An inquiry that asks questions of individuals drawn from a population (such as a poll)
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Census
A sample that consists of the entire population
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Population
The entire group about whom a statistician hopes to learn
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Sample
A portion of a population examined to learn about that population
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Population Parameter
A numerical attribute for an entire population (rarely know this)
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Sample Statistic
A numerical attribute calculated for sampled data and used to *infer* the associated population parameter
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Sample Size
The number of individuals, n, in a sample
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Randomization
Each individual is given a fair, random chance of selection
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Representative
The sample statistic(s) computed from a sample accurately reflect(s) the corresponding population parameter(s)... you took a "good sample"
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Simple Random Sampling (SRS)
Each combination of n individuals in a population has an equal chance of selection... often done by assigning a number to each individual and generating n random numbers
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Stratified Random Sampling
Sampling method where the population is divided into several homogeneous subsets (called strata) and THEN random samples are proportionally drawn from each stratum
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Cluster Sampling
Sampling method where entire groups or "clusters" or subjects are selected at random; the clusters must be heterogeneous and individually representative of the population
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Systematic Sampling
Sampling method in which subjects are listed as one subject is selected, followed by each of the next nth subjects
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Sampling Variability
The natural tendency of randomly drawn samples to differ from one another
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Sampling Bias
Systematic failure of a sampling method to properly represent its population
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Voluntary Response Bias
Individuals must volunteer or take some prior action to be included in a sample
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Convenience Sampling Bias
Subjects chosen for a sample are simply those conveniently available due to some factor
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Undercoverage Bias
Some portion of the population is not sampled at all or has a reduced chance of being included in the sample
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Nonresponse Bias
A substantial fraction of those properly sampled fail to respond
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Response Bias
Anything in a survey design that influences the responses, such as wording survey questions in a way that suggests a favored response or perceived pressure in the survey
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Observational Study
Study in which the researchers do not assign treatments, they simply observe them
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Retrospective Study
Observational study in which subjects are selected and then their previous conditions or behaviors are observed
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Prospective Study
Observational study in which subjects are selected and then followed to observe future outcomes
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Experiment
An experimenter controls factor levels to create treatments, randomly assigns subjects to those treatments, replicates the experiment with all of those subjects, and then compares the responses of the subjects
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Completely Randomized Design
Every subject and combination of subjects has an equal chance of receiving each treatment
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Factor
An explanatory variable whose levels are controlled by an experimenter
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Response Variable
The variable whose resulting values are compared across different treatments
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Level
The specific value that the experimenter chooses for a factor
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Treatment
The controlled circumstances applied to experimental groups, made of different levels of a single factor or combinations of two or more factor levels
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Control Group
A baseline treatment level that provides a basis for comparison; often either a well-understood level or a null treatment
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Subject/Experimental Unit
Individuals or objects on whom an experiment is performed
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Randomization
An experiment must assign subjects to treatment groups at random
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Replication
Each treatment must be repeated for multiple subjects
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Statistically Significant
When an observed difference is too large to believe that is was likely to have occurred naturally
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Single-Blind
When either the subjects themselves or the administrators/evaluators in an experiment are unaware of which subjects were assigned to each treatment group
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Double-Blind
When both subjects themselves and the administrators/evaluators in an experiment are unaware of which subjects were assigned to each treatment group
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Placebo
A treatment known to have no effect, administered to the control group so that all groups experience the same condition (such as a sugar pill)
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Placebo Effect
A perceived reaction to an ineffective treatment... mist compare to a placebo to be sure that the observed effect of a treatment is not simply due to the placebo effect
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Blocking
Grouping subjects together by uncontrollable attributes, thus allowing an experimenter to isolate the variability attributable to differences in the blocks and see the differences caused by the treatments more clearly
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Randomized Block Design
Experimental design in which randomization occurs only within blocks; treatment groups are not mixed for different blocks
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Confounding Variable
When the levels of one factor are associated with the levels of another factor and this their effects cannot be separated
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Matched Pairs Design
Experimental design where the same individual for multiple treatment trials or individuals with similarities not under study are paired together when undergoing the experiment, hopefully reducing the variation in the data from other factors
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