Starnes/Tabor, The Practice of Statistics for the AP® Course, 7e, Unit 3, English

population

In a statistical study, the entire group of individuals we want information about.

census

Study that collects data from every individual in the population.

sample

Subset of individuals in the population from which we collect data.

sample survey

Study that collects data from a sample to learn about the population from which the sample was selected.

random sampling

Using a chance process to determine which members of a population are chosen for the sample.

observational study

Study that observes individuals and measures variables of interest but does not attempt to influence the responses.

retrospective observational study

An observational study that uses existing data for a sample of individuals.

prospective observational study

An observational study that tracks individuals into the future.

experiment

A study in which researchers deliberately impose treatments on experimental units to measure their responses.

simple random sample (SRS)

Sample 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 with replacement

When an individual from a population can be selected more than once when choosing a sample.

sampling without replacement

When an individual from a population can be selected only once when choosing a sample.

strata

Groups of individuals in a population that share characteristics thought to be associated with the variables being measured in a study. The singular form of strata is stratum.

stratified random sample

A sample selected by dividing the population into non-overlapping groups (strata) of individuals that share characteristics thought to be associated with the variables being measured in a study, selecting 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 sample

A sample selected by dividing the population into non-overlapping groups (clusters) of individuals that are located near each other, randomly choosing clusters, and including each member of the selected clusters in the sample.

systematic random sample

A sample selected by choosing individuals from an ordered arrangement of the population by randomly selecting one of the first k individuals and choosing every kth individual thereafter.

convenience sample

A sample that consists of individuals from the population who are easy to reach. Convenience sampling leads to bias when the members of the sample differ from the population in ways that affect their responses.

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 sample

A sample that consists of people who choose to be in the sample by responding to a general invitation. Voluntary response samples are sometimes called self-selected samples. Voluntary response sampling leads to voluntary response bias when the members of the sample differ from the population in ways that affect their responses.

undercoverage

Occurs when some members of the population are less likely to be chosen or cannot be chosen in a sample. Undercoverage bias occurs when the underrepresented individuals differ from the population in ways that affect their responses.

nonresponse

Occurs when an individual chosen for the sample can’t be contacted or refuses to participate. Nonresponse bias occurs when the individuals who can’t be contacted or who refuse to participate differ from the population in ways that affect their responses.

response bias

Occurs when there is a consistent pattern of inaccurate responses to a survey question. Includes bias due to question wording.

response variable

Variable that measures the outcome of a study.

explanatory variable

Variable that may help predict or explain changes in a response variable.

confounding

When two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.

confounding variable

A variable related to both the explanatory variable and the response variable in a study that may create the false impression of a cause-and-effect relationship between the explanatory and response variables.

treatment

Specific condition applied to the 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. When the experimental units are human beings, they are often called subjects.

subjects

Experimental units that are human beings.

placebo

A treatment that has no active ingredient but is otherwise like other treatments.

factor

Explanatory variable in an experiment that is manipulated and may cause a change in the response variable.

level

Specific value of an explanatory variable (factor) in an experiment.

control group

Experimental group whose primary purpose is to provide a baseline for comparing the effects of the 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 (placebo).

double-blind

An experiment in which neither the subjects nor those who interact with them and measure the response variable know which treatment a subject is receiving.

single-blind

An experiment in which 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

Experimental design principle. Use of a chance process to assign experimental units to treatments (or treatments to experimental units). Doing so helps create roughly equivalent groups of experimental units by balancing the effects of other variables among the treatment groups, allowing for cause-and-effect conclusions.

completely randomized design

Design in which the experimental units are assigned to the treatments completely at random.

replication

Experimental design principle. 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.

control

Experimental design principle. Keeping variables (other than the explanatory variable) the same for all groups, especially variables that are likely to affect the response variable. Helps avoid confounding and reduces variability in the response variable.

block

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

Experimental design that forms groups (blocks) consisting of individuals that are similar in some way that is expected to affect the response to the treatments and then randomly assigns experimental units to treatments separately within each block.

matched pairs design

Common experimental design for comparing two treatments that uses blocks of size 2. In some matched pairs designs, each subject receives both treatments in a random order. In others, two very similar experimental units are paired and the two treatments are randomly assigned within each pair.

statistically significant

In an experiment, when the difference in responses between the groups is so large that it is unlikely to be explained by the chance variation in the random assignment, the results are called statistically significant.