Section 4.2

Experiment

An experiment deliberately imposes some treatment on individuals to measure their responses. - When our goal is to understand cause and effect, experiments are the only source of fully convincing data. - Experiments can give good evidence for causation.

Experimental Units

The experimental units are the smallest collection of individuals to which treatments are applied.

Control Group

The primary purpose of a control group is to provide a baseline for comparing the effects of the other treatments.

Principles of Experimental Design

1) Control for lurking variables that might affect the response: Use a comparative design and ensure that the only systematic difference between the groups is the treatment administered. 2) Random assignment: Use impersonal chance to assign experimental units to treatments. This helps create roughly equivalent groups of experimental units by balancing the effects of lurking variables that aren't controlled on the treatment groups. 3) Replication: 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.

Block

A block 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.

Comparative Experiment

A comparative experiment is an experiment that compares two or more treatments. Often times, the remedy for confounding in an experiment/study is to conduct a comparative experiment.

Matched Pairs Design

A form of randomized block design for comparing exactly two treatments by matching pairs of similar experimental units (e.g. twins). - Chance is used to decide which member of a pair gets the first treatment. Then the other subject in that pair gets the other. - Sometimes each "pair" in a matched pairs design consists of just one experimental unit that gets both treatments one after the other. In that case, each experimental unit serves as its own control. Also, the order of the treatments can influence the response, so we randomize the order for each experimental unit.

Lurking Variables

A lurking variable is a variable that is not among the explanatory or response variables in a study but may influence the response variable.

Treatment

A specific condition applied to the individuals in an experiment is called a treatment. If an experiment has several explanatory variables, a treatment is a combination of specific values of these variables.

Single-Blind

An experiment can still be single-blind if the individuals who are interacting with the subjects and measuring the response variable don't know how a treatment may effect each subject. - In other single-blind experiments, the subjects are unaware of which treatment they are receiving, but the people interacting with them and measuring the response variable do know.

Placebo Effect

An experimental unit's response to a "dummy" treatment, a placebo, is called the placebo effect.

Observational Study

An observational study observes individuals and measures variables of interest but does not attempt to influence the responses. - Observational studies of the effect of one variable on another often fail because of confounding between the explanatory variable and one or more lurking variables.

Statistically Significant

An observed effect so large that it would rarely occur by chance is called statistically significant.

Confounding

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

Completely Randomized Design

In a completely randomized design, the treatments are assigned to all the experimental units completely by chance.

Double-Blind

In a double-blind experiment, neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received.

Randomized Block Design

In a randomized block design, the random assignment of experimental units to treatments is carried out separately within each block. - Blocks act as another form of control. They control the effects of some outside variables by bringing those variables into the experiment to form the blocks. ** Control what you can, block what you can't, and randomize to create comparable groups.

Random Assignment

In an experiment, random assignment means that experimental units are assigned to treatments at random, that is, using some sort of chance process.

Subjects

When the experimental units are human beings, they often are call subjects.