Statistics chapter 13

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experiments and observational studies

experiment

an experiment 'manipulates' factor levels to create treatments, 'randomly' assigns subjects to these treatments levels, and then 'compares' the responses of the subject groups across treatment levels

random assignment

to be valid, an experiment must assign experimental units to treatment groups at random. this is called random assignment

factor

a variable whose levels are controlled by the experimenter; experiments attempt to discover the effects that differences in factor levels may have on the responses of the experimental units

response

a variable whose values are compared across different treatments. In a randomized experiment, large response differences can be attributed to the effect of differences in treatment level

experimental unit

individuals on whom an experiment is performed; usually called 'subjects' or 'participants' when human

level

specific values that the expeerimenter chooses for a factor are called the levels of the factor

treatment

the process, intervention or other controlled circumstance applied to randomly assigned experimentals units. treatments are the differnt levels of a single factor or are made up of combinations of levels of two ore more factors

principles of expermental design

control, randomize, replicate, and block

control

control aspects of the experiment that we know may have an effect on the response, but that are not the factors being studied

randomize

randomize subjects to treatments to even out the effects that we cannot control

replicate

replicate over as many subjects as possible. results for a single subject are just anecdotes; if the subjects are not a representative sample from the population of intereest, replicate the entire study with a different group of subjects, preferably from a different part of the population

block

block to reduce the effects of identifiable attributes of the subjects that cannot be controlled

statistically significant

when an observed difference is too large for us to believe that it is likely to have occurred naturally, we consider the difference to be statistically significant

control group

the experimental units assigned to a baseline treatment level, typically either the default treatment, which is well understood or null, placebo treatment. their responses provide the basis for comparison

blinding

any individual associated with an experiment who is not aware of how subjects have been allocated to treatment groups is said to be blind

single-blind

when every individual in either the 'influence' or 'evaluation' class is blinded its single | those who could 'influence the results' ; ex. the subjects, treatment administrators, or technicians

double blind

when everyone in the 'influence' and 'evaluation' classes is blinded its double | those who 'evaluate the results'; ex. judges, treating physicians, etc

placebo

a treatment known to have no effect.

placebo effect

the tendency of many human subjects [ often 20% ] to show a response even when administred a placebo

matching

matching, like blocking reducdes unwanted variation; in a retrospective or prospective study, subjects who are similar in ways not under study may be matched and then compared with each other on the variables of interest

designs

randomized block design and completely randomized design

confounding

confusing or surprising results; when the levels of one factor are associated with the levels of another factor so their effects cannot be separated, we say that these two factors are confounded

randomized block design

the randomization occurs only within blocks

completly randomized design

all experimental units have an equal chance of receiving any treatment

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