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Designing ExperimentsChapter 5 Section 1a

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Variables Response

The variable we are studyingby the outcome of an

experiment Also called dependent

variable

Explanatory

Helps explain or influences

changes in the responsevariable

Also called independentvariable

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Basic Vocab of Experiments

Experimental Units

The individuals on which the experiment is done

CalledSUBJECTSif the units are humans

Treatment The specific experimental condition applied to the

units

Factors

The explanatory variables of an experiment

Level

A specific value of the factor(s)

See Example 5.14 (page 355)

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More Vocab (Ex. 5.15 pg. 356) Basic design of a single treatment experiment

Treatment Observe Response

P

laceboE

ffect When a dummy treatment is as effective as theactual treatment

This is caused by outside factors

Control Group A group of experimental units that receive a

placebo or no treatment, but experience thesame conditions as the treatment group

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#5.34 Identify the experimental units, factors,treatments, and the response variables

You can use your computer to make long-distance telephone calls over the Internet. How

will the cost affect the use of this service? Auniversity plans an experiment to find out. It

will offer the service to all 350 students in one ofits dormitories. Some students will pay a low

flat rate. Others will pay higher rates at peakperiods and very low rates off-peak. Theuniversity is interested in the amount and timeof use and in the effect on the congestion of the

network.

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Basic Principles of Statistical Design ofExperiments1. Control

2. Replication3. Randomization

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Control The overall effort to minimize the effects of

lurking variables on the response (Dont confusew/ Control Group)

Comparison is the simplest form of control. Compare two or more treatments (Control Group

& Treatment Group) in order to preventconfounding the effect of a treatment with otherinfluences, such as lurking variables.

Think Gastric Freezing Example (pg. 358)

Uncontrolled experiments usually show a muchhigher success rate than proper comparative

experiments

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Replication

Use enough subjects to reduce (it cannot

eliminate) chance variation This DOES NOT mean repeat the same

experiment a 2nd time in this instance.

Use enough Experimental Units (or subjects)

and the effects of chance will even out

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Randomization

Use impersonal chance to assign experimental

units to treatments. Systematic differences among groups of

experimental units in a comparative experimentcause biaschance eliminates this!

Randomization allows us to assume that thetreatment groups are essentially similar.

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Diagramming Experiments

#5.44 on page 366What to show: randomization, size of groups, treatments,

response variable

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Statistical Significance

An observed effect

so large that itwould rarely occurby chance.

Think back to

Gastric FreezingExample (pg. 356)

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Pg. 357 5.33-5.37 oddPg. 364 5.39-5.43 oddHomework Assignment

Read pages 358-371