Chapter 5.2 Designing experiments

Terminology The individuals on which the

experiment is done are the experiment units.

When the units are human beings they are called subjects.

A specific experiment conditions applied to the units is called the treatment.

Comparative Experiments

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Units Treatment Observe Response

Why is this not a greatdesign?• Lack of a control group

More terminology The explanatory variable in an

experiment is often called factor(s).

Each treatment is formed by combining a specific value (often called a level) of each of the factors.

The Physicians’ Health Study

Ex: Does regularly taking aspirin help protect people against heart attacks? The Physicians’ Health Study looked at the effects of two drugs: aspirin and beta carotene. The body converts beta carotene into vitamin A, which may help prevent some forms of cancer. A combination of the drugs were given to 21,996 male physicians.

The Physicians’ Health Study Subjects?

◦Physicians Factors?

◦2 (Aspirin &Beta carotene)Treatments?

◦4 groupsAspirin, Beta carotene

Aspirin, Placebo

Placebo, Beta carotene

Placebo, Placebo

The Placebo Effect “Gastric freezing” is a clever

treatment for ulcers in the upper intestine. The patient swallow a deflated balloon with tubes attached, then a refrigerated liquid is pumped through the balloon for an hour. The idea is that cooling the stomach will reduce its production of acid and so relieve ulcers. Comparative Experiment

Units Treatment Observe Response

The Placebo Effect The “Gastric freezing”

experiment was poorly designed. The patients’ response may have been due to the placebo effect. A placebo is a dummy treatment. Many patients respond favorably to any treatment, even placebo.

Placebo UsePeople who receive the placebo

are members of the control group.

People who receive the “real” treatment are in the treatment group.

Practice5.31-5.34…

Randomized Comparative Experiments

Units(random assignment)

Group 1

Group 2

Compare Response

Group 3

Group 4

Example: Health StudyDoes regularly taking aspirin help protect

people against heart attacks? The Physicians’ Health Study looked at the effects of two drugs: aspirin and beta carotene. The body converts beta carotene into vitamin A, which may help prevent some forms of cancer. A combination of the drugs were given to 21,996 male physicians.

Generating random Numbers w/calculator “Plant a common seed” for

common answers ◦115rand

rand is in MATH, PRB

◦randInt(1,31)

Principles of Experiment Design Control the effects of lurking

variables on the response, most simply by comparing two or more groups.

Randomize-use impersonal chance to assign experimental units treatments.

Replicate each treatment on many units to reduce chance variation in the results.

Statistical Significance An observed effect so large that

it would rarely occur by chance is called statistically significant.

Double-Blind Experiment

Blind Experiment ◦Only the people in the experiment

don’t know what they received..

In a double-blind experiment, neither the subjects nor the people who have contact with the subjects know which treatment a subject received.

Experiments w/out placebos Matched pair design

◦Subjects are paired by matching common important attributes.

◦Often the results are a pre-test and post-test with the unit being “matched” to itself.

Example◦Traditional vs. Block Schedule ◦Similar schools ◦1 school traditional ◦1 school block

Compare at the end using pre/post test

Block Design A block is a group of experimental units

or subjects that are known before the experiment to be similar in some way that is expected to affect the response to the treatments. In a block design, the random assignment of units to treatments is carried out separately within each block.

Stratified sampling used in random sampling

Block Design Experimental

Block Design Units

Males

Aspirin

Placebo

Compare

Females

Aspirin

Placebo

Subjective

split

Randomsplit

to treatment groups

Assignment Exercises : 5.31-5.34…5.35-

5.51odd