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AP Statistics Chapter 13- Experiments and Observational Studies

Terms in this set (21)

observational study
a study based on data in which no manipulation of factors has been employed
retrospective study
an observational study in which subjects are selected and then their previous conditions or behaviors are determined. Retrospective studies need not be based on random samples and they usually focus on estimating differences between groups or associations between variables
prospective study
an observational study in which subjects are followed to observe future outcomes. Because no treatments are deliberately applied, a prospective study is not an experiment. Nevertheless, prospective studies typically focus on estimating differences among groups that might appear as the groups are followed during the course of the study.
experiment
an experiment manipulates factor levels to create treatments, randomly assigns subjects to these treatment 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
factor
a variable whose levels are manipulated by the experimenter. Experiments attempt to discover the effects that differences in factor levels may ave on differences in treatment level
response
a variable whose values are compared across different treatments. In a randomized experiment, large response differences can be attributed to the effect or differences in treatment level.
experimental units
individuals on whom an experiment is performed. usually called subjects or participants when they are human
level treatment
the specific values that the experimenter chooses for a factor
treatment
the process, intervention, or other controlled circumstance applied to randomly assigned experimental units. Treatments are the different levels of a single factor or are made up of combinations of levels of two or more factors
principles of experimental design
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 effects that we cannot control

Replicate over as many subjects as possible. Results for a single subject are just anecdotes. If, as often happens, the subjects of the experiment are not a representative sample from the population of interest, replicate the entire study with a different group of subjects, preferable 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 a null, placebo treatment. Their responses provide a 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 blinded
single-blind/double-blind
There are two main classes of individuals who can affect the outcome of an experiment: those who influence the results (subjects, treatment administrators, or technicians) and those who evaluate the results (judges, treating physicians, etc.)

When every individual in either of these classes is blinded, an experiment is said to be single-blind.
When everyone in both classes is blinded, we call the experiment double-blind.
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