AP Statistics Chapter 13- Experiments and Observational Studies

a study based on data in which no manipulation of factors has been employed

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

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.

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.

to be valid, an experiment must assign experimental units to treatment groups at random

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

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.

individuals on whom an experiment is performed. usually called subjects or participants when they are human

the specific values that the experimenter chooses for a factor

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

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.

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

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

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

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.

a treatment known to have no effect, administered so that all groups experience the same conditions. Many subjects respond to such a treatment (a response known as a placebo effect). Only by comparing with a placebo can we be sure that the observed effect of a treatment is not due to simply the placebo effect.

the tendency of many human subjects (often 20% or more of experiment subjects) to show a response even when administered a placebo

when groups of experimental units are similar, it is often a good idea to gather them together into blocks. By blocking, we isolate the variability attributable to the differences between the blocks so that we can see the differences caused by the treatments more clearly.

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. Matching, like blocking, reduces unwanted variation.

in a completely randomized design, all experimental units have an equal chance of receiving any treatment. In a randomized block design, the randomization occurs only within blocks.

when the levels of one factor are associated with the levels in another factor in such a way that their effects cannot be separated, we cay that these two factors are confounded.