24 terms

Statistics chapter 13

experiments and observational studies
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
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
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
specific values that the expeerimenter chooses for a factor are called the levels of the factor
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 aspects of the experiment that we know may have an effect on the response, but that are not the factors being studied
randomize subjects to treatments to even out the effects that we cannot control
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 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
any individual associated with an experiment who is not aware of how subjects have been allocated to treatment groups is said to be 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
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, 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
randomized block design and completely randomized design
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