Experiments

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

measures an outcome of a study

may help explain or predict changes in a response variable

occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other

deliberately imposes treatments (conditions) on individuals to measure their response

a treatment that has no active ingredient, but is otherwise like other treatments

a specific condition applied to the individuals in an experiment

the object to which a treatment is randomly assigned

when experimental units are human beings

an explanatory variable that is manipulated and may cause a change in the response variable

the different values of a factor

is used to provide a baseline for comparing the effects of other treatments and depending on the purpose of the experiment, a control group may be given an inactive treatment (placebo), an active treatment, or no treatment at all

describes the fact that some subjects in an experiment will respond favorably to any treatment, even an inactive experiment

neither subjects nor those who interact with them and measure the response variable know which treatment a subject is receiving

means that experimental units are assigned to treatments using a chance process

keeping other variables constant for all experimental units

means giving each treatment to enough experimental units so that a difference in the effects of the treatments can be distinguished from chance variation due to random assignment

the experimental units are assigned to the treatment completely at random

is 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

the random assignment of experimental units to treatments is carried out separately within each block

a common experimental design for comparing two treatments that uses blocks of size 2. IN some matched pairs designs, two very similar experimental units are paired and the two treatments are randomly assigned within each pair. In others, each experimental unit receives both treatments in a random order