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