Anonymity
When the names of individuals participating in a study are not known even to the director of the study.
Bias
The design of a statistical study systematically favors certain outcomes.
Block
A group of experimental units known before the experiment to be similar in some way that is expected to affect response.
Census
A study that attempts to collect data from every individual in the population.
Cluster sample
All individuals in the chosen group are included in the sample.
Completely randomized design
When the treatments are assigned to all the experimental units completely by chance.
Confidentiality
A basic principle of data ethics that requires individual data to be kept private.
Confounding
When two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.
Control
Done by using a comparative design and ensuring the only systematic difference between the groups is the treatment.
Control group
An experimental group whose primary purpose is to provide a baseline for comparing the effects of the other treatments.
Convenience sample
A sample selected by taking the members of the population that are easiest to reach
Double-blind
An experiment in which neither the subjects nor those who interact with them know which treatment a subject received.
Experiment
Deliberately imposes some treatment on individuals to measure their responses.
Experimental units
Individuals to which treatments are applied.
Explanatory variable
A variable that helps explain or influences changes in the other variable.
Factor
Another name for the explanatory variable in an experiment
Inference
Using information from a sample to draw conclusions about the larger population.
Inference about cause and effect
Using the results of an experiment to conclude that the treatments caused the difference in responses.
Informed consent
Individuals must be made aware in advance about the nature of a study and any risk of harm, then agree in writing.
Institutional review board
All planned studies must be approved in advance and monitored to protect the safety and well-being of the participants.
Lack of realism
When the treatments, the subjects, or the environment of an experiment are not realistic.
Level
A specific value of an explanatory variable (factor) in an experiment.
Lurking variable
A variable that is not among the explanatory or response in a study but that may influence the response variable.
Margin of error
A numerical estimate of how far the sample result is likely to be from the truth about the population due to sampling variability.
Matched pair
A common form of blocking for comparing just two treatments.
Nonresponse
Occurs when a selected individual cannot be contacted or refuses to cooperate
Nonsampling error
Some common examples are nonresponse, response bias, and errors due to question wording.
Observational study
Observes individuals and measures variables of interest but does not attempt to influence the responses.
Placebo
An inactive (fake) treatment.
Placebo effect
Describes the fact that some subjects respond favorably to any treatment, even an inactive one.
Population
In a statistical study, the entire group of individuals about which we want information.
Random assignment
Use some chance process to assign experimental units to treatments.
Random sampling
The use of chance to select a sample
Randomized block design
Random assignment of treatments is carried out separately within each block.
Replication
Use enough experimental units in each group so that any differences in the effects of the treatments can be distinguished from chance differences between the groups.
Response bias
A systemic pattern of incorrect responses.
Response variable
A variable that measures an outcome of a study.
Sample
The part of the population from which we actually collect information.
Sampling error
Mistakes made in the process of taking a sample. Bad sampling methods and undercoverage are common types.
Sampling frame
The list from which a sample is actually chosen.
Simple random sample (SRS)
Gives every possible sample of a given size the same chance to be chosen.
Single-blind
An experiment in which either the subjects or those who interact with them, but not both, know which treatment a subject received.
Statistically significant
An observed effect so large that it would rarely occur by chance.
Strata
Groups of individuals in a population that are similar in some way that might affect their responses.
Stratified random sample
First classify the population into groups of similar individuals then choose a separate SRS from each group to form the full sample.
Subjects
Experimental units that are human beings.
Table of random digits
A long string of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Treatment
A specific condition applied to the individuals in an experiment.
Undercoverage
Occurs when some members of the population are left out of the sampling frame
Wording of questions
The most important influence on the answers given to a survey.