Observational study

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

Experiment

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

Response variable

Measures an outcome of a study

explanatory variable

may help explain or predict changes in a response variable

confounding

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

treatment

a specific condition applied to the individuals in an experiment

placebo

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

experimental unit

the object to which the treatment is randomly assigned

Subject

experimental units that are human beings.

factor

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

levels

different values of a factor

control group

used to provide a baseline for comparing the effects of other treatments in an experiment

placebo effect

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

random assignment

Experimental units are assigned to treatments using a chance process.

control

keeping other variables constant for all experimental units

replication

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

Completely randomized design

the experimental units are assigned to the treatments completely at random

Double blind

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

Single blind

either the subjects or the people who interact with them and measure the response variable don't know which treatment a subject is receiving

Block

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

Randomized block design

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