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Designing ExperimentsChapter 5 Section 1a
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Variables
Response The variable we are studying
by the outcome of an
experiment Also called dependent variable
Explanatory Helps explain or influences
changes in the response variable
Also called independent variable
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Basic Vocab of Experiments Experimental Units
The individuals on which the experiment is done Called SUBJECTS if the units are humans
Factors The explanatory variables of an experimentTreatment
The specific experimental condition applied to the units
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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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More Vocab (Ex. 5.15 pg. 356) Basic design of a single treatment experiment
Treatment Observe Response Placebo Effect
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 aplacebo or no treatment, but experience thesame conditions as the treatment group
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Basic Principles of Statistical Design ofExperiments1. Control2. Replication3. Randomization
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Control The overall effort to minimize the effects of
lurking variables on the response (Dont confuse w/ 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 much
higher success rate than proper comparative
experiments
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Replication Use enough subjects to reduce (it cannot
eliminate) chance variation & bias This DOES NOT mean repeat the same
experiment a 2 nd time in this instance. Would you expect to get the exact same results if
you did?
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Randomization Use impersonal chance (probability) to assign
experimental units to treatments. Systematic differences among groups of
experimental units in a comparative experimentcause biaschance (probability) 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 it would rarely occur by 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