Observational Study
Observes individuals and measures variables of interest but does not attempt to influence the responses.
Experiment
Deliberately imposes some treatment on individuals to measure their responses. Only source of fully convincing data for cause and effect.
Lurking Variable
A variable that is not among the explanatory or response variables in a study but may influence the response variable.
Confounding
When two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other.
Experimental Units
The smallest collection of individuals to which treatments are applied. Ex: people, animals, or objects.
Subjects
Experimental units that are human beings.
Treatment
A specific experimental condition applied to the units. Ex: what you do to plants in order to observe a response.
Explanatory Variable
a variable that we think explains or causes changes in the response variable
Response Variable
a variable that measures an outcome or result of a study
Factors
Explanatory variable in experiments.
Random Assignment
An important experimental design principle. The experimental units are assigned to treatments at random, that is, using some sort of chance process. Helps create roughly equivalent groups by balancing the effects of lurking variables that aren't controlle
Completely Randomized Design
The treatments are assigned to all the experimental units completely by chance.
Control Group
Provides a baseline for comparing the effects of the other treatments. A control group may be given a placebo or an active treatment.
Placebo
An inactive (fake) treatment.
Control
An important experimental design principle. Control for lurking variables that might affect the response by using a comparative design and ensuring that the only systematic difference between the groups is the treatment administered.
Replication
An important experimental design principle. Use enough experimental units in each group so that any differences in the effects of the treatments can be distinguished from chance differences between the groups.
Placebo Effect
Describes the fact that some subjects respond favorably to any treatment, even an inactive one (placebo).
Double Blind
An experiment in which neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received.
Single Blind
An experiment in which either the subjects or those who interact with them and measure the response variable, but not both, know which treatment a subject received.
Statistically Significant
An observed effect so large that it would rarely occur by chance.
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. Not formed at random. Ex: 2 blocks - light colored and dark-colored pieces of laundry.
Randomized Block Design
Start by forming blocks consisting of individuals that are similar in some way that is important to the response. Random assignment of treatments is then carried out separately within each block.
Matched Pairs
A common form of blocking for comparing just two treatments. In some matched pairs designs, each subject receives both treatments in a random order. In others, the subjects are matched in pairs as closely as possible, and each subject in a pair is randoml