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the design of a statistical study shows bias if it systematically favors certain outcomes


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


a study that attempts to collect data from every individual in the population

Cluster sample

divide the population into smaller groups. These clusters should mirror the characteristics of the population. Then choose an SRS of the clusters. All individuals in the chosen clusters are included in the sample.

Completely randomized design

when the treatments are assigned to all the experimental units completely by chance


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


controlling for as many lurking vars as possible, by using a comparative design and making sure the only difference between the groups is the treatment administered.

Control group

a experimental group that serves as a baseline for comparing the effects of other treatments. may be a placebo, commonly used to measure the difference between psychological and physical effects.

convenience sample

a sample selected by taking the members of the population


an experiment in which neither the subject nor the administrator that measures the response variable know which treatment a subject received


deliberately imposes some treatment on individuals to measure their responses

experimental units

person, a plot of land, a machine, etc. that forms the 'material' on which an experiment is performed.

explanatory variables

a variable that helps explain or influences changes in a response variable


the explanatory variables in an experiment


specific value of an explanatory variable(factor) in an experiment

lurking variable

a variable that is not among the explanatory or response variables in a study but that may influence the response variable.

matched pair

a form of blocking for comparing just two treatments. In some, each subject receives both treatments in a random order. In others, the subjects are matched in pairs as closely as possible, and each subject in a pair is randomly assigned to receive one of the treatments

non response

when a selected individual can't be contacted or refuses to cooperate, example of a nonsampling error

observational study

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


an inactive (fake) treatment

placebo effect

describes the fact some subjects respond favorably to any treatment(psychological effect)


entire group of individuals from which we want information

random assignment

assign experimental units to treatments by chance

random sampling

use of chance to select a sample

randomized block design

forming blocks consisting of similar individuals. random assignment of treatments


use enough experimental units in each group so that any differences in the effects of the treatments

response bias

a systematic pattern of incorrect responses

response variable

a variable that measures an outcome of a study


the part of the population from which we collect information. we use information from a sample to draw conclusions about an entire population

sampling error

mistakes while taking a sample, that lead to inaccurate information about a population

SRS(simple random sample)

gives every possible sample the same chance to be chosen.

stratified random sample

classify the population into similar individuals , then get an SRS from each strata


experimental units that are human beings


specific condition applied to individuals in an experiment


occurs when some members of the population are left out of the sampling; a type of sampling error

voluntary response samples

people decide whether to join a sample; biased by people with strong opinions

wording of questions

confusing or leading questions can lead to strong bias

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