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population
In a statistical study, is the entire group of individuals we want information about.
census
Collects data from every individual in the population.
Sample
A subset of individuals in the population from which we collect data.
sample survey
A study that collects data from a sample to learn about the population from which the sample was selected.
Convenience sampling
Selects individuals from the population who are easy to reach.
Bias
The design of a statistical study shows bias if it is very likely to underestimate or very likely to overestimate the value you want to know.
Voluntary response sampling
Allows people to choose to be in the sample by responding to a general invitation.
Random sampling
Involves using a chance process to determine which members of a population are included in the sample.
Simple random sample (SRS)
A simple random sample (SRS) of size n is chosen in such a way that every group of n individuals in the population has an equal chance to be selected as the sample.
sampling without replacement
When an individual from a population can be selected only once.
sampling with replacement
When an individual from a population can be selected more than once.
Strata
Groups of individuals in a population who share characteristics thought to be associated with the variables being measured in a study.
Stratified random sampling
Selects a sample by choosing an SRS from each stratum and combining the SRSs into one overall sample.
Cluster
A group of individuals in the population that are located near each other.
Cluster sampling
Selects a sample by randomly choosing clusters and including each member of the selected clusters in the sample.
Systematic random sampling
Selects a sample from an ordered arrangement of the population by randomly selecting one of the first k individuals and choosing every k th individual thereafter.
Undercoverage
Occurs when some members of the population are less likely to be chosen or cannot be chosen in a sample.
Nonresponse
Occurs when an individual chosen for the sample can't be contacted or refuses to participate.
Response bias
Occurs when there is a systematic pattern of inaccurate answers to a survey question.
observational study
Observes individuals and measures variables of interest but does not attempt to influence the responses.
response variable
Measures an outcome of a study.
explanatory variable
May help explain or predict changes in a response variable.
Confounding
Occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.
experiment
Deliberately imposes treatments (conditions) on individuals to measure their responses.
placebo
A treatment that has no active ingredient, but is otherwise like other treatments.
treatment
A specific condition applied to individuals in an experiment. If an experiment has several explanatory variables, a treatment is a combination of specific values of these variables.
experimental unit
The object to which a treatment is randomly assigned.
subjects
A term often used when experimental units are human beings.
factor
In an experiment, is an explanatory variable that is manipulated and may cause a change in the response variable.
levels
The different values of a factor.
control group
In an experiment, a control group is used to provide a baseline for comparing the effects of other treatments. Depending on the purpose of the experiment, a control group may be given an inactive treatment (placebo), an active treatment, or no treatment at all.
placebo effect
Describes the fact that some subjects in an experiment will respond favorably to any treatment, even an inactive treatment.
double-blind
neither the subjects nor those who interact with them and measure the response variable know which treatment a subject is receiving.
single-blind
either the subjects or the people who interact with them and measure the response variable don't know which treatment a subject is receiving.
random assignment
In an experiment, means that experimental units are assigned to treatments using a chance process.
control
In an experiment, means keeping other variables constant for all experimental units.
replication
In an experiment, means giving each treatment to enough experimental units so that a difference in the effects of the treatments can be distinguished from chance variation due to the random assignment.
completely randomized design
In a completely randomized design, the experimental units are assigned to the treatments completely at random.
block
A group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments.
randomized block design
The random assignment of experimental units to treatments is carried out separately within each block.
matched pairs design
A common experimental design for comparing two treatments that uses blocks of size 2. In some matched pairs designs, two very similar experimental units are paired and the two treatments are randomly assigned within each pair. In others, each experimental unit receives both treatments in a random order.
Sampling variability
Refers to the fact that different random samples of the same size from the same population produce different estimates.
statistically significant
When the observed results of a study are too unusual to be explained by chance alon
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