Experimental Design. Experiment Experiment: Researchers impose some change (treatment) and measure...
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Transcript of Experimental Design. Experiment Experiment: Researchers impose some change (treatment) and measure...
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Experimental Design
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Experiment• Experiment: Researchers impose some
change (treatment) and measure the result or response.
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Principles of Experiment Design• Randomization: unknown and uncontrollable
difference are handled by randomizing who receives what treatments.
• Replication: treatments need to be repeated on a sufficient number of subjects.
• Generalizability: ability to repeat an experiment in a variety of settings.
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Terminology• Experimental Units: the individuals/items on
which the experiment is done – When the units are human beings, they are called
subjects.
• Treatment: A specific experimental condition applied to the unit. Can be multiple treatments in one experiment.
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Factors & Level• Factors: the explanatory variables in an
experiment • Level: the specific level/value of each factor. – Example: – Factor: Sleep. – Level: 6 hours, 7 hours and 8 hours of sleep.
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The Physicians’ Health Study
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. Name the subjects, treatments and factors?
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The Physicians’ Health Study
• Subjects?– Physicians
• Treatments?– 4 (the groups->
• Factors?– 2 (aspirin & beta carotene)
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Lurking Variables• Lurking variables: a variable that drive two
other variables, creating the mistaken impression that the two other variables are related by cause and effect.
• Example: As shoe size increases, reading scores increase….what is the lurking variable?
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Placebo v. Treatment• People who receive the placebo are members
of the control group.• People who receive the “real” treatment are
the treatment group.
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Specific Types of Experiments
• Double-Blind• Single-Blind• Matched Pairs• Block Design
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Double-Blind• In a double-blind experiment, neither the
subjects nor the experimenters know which treatment a subject received.
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Matched Pair Design• In a matched pair design, subjects are paired
by matching common important attributes.• Some times the results are a pre-test and
post-test with the unit being “matched” to itself.
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Matched Pair Design• Example: – Tire wear and tear.– Put one set of tires on the left side of the car and
a different set on the right side of the car. – This would help control the lurking variable of
different driving styles (between teenage boys and teachers) and mileage driven.
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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.
• Helps control for lurking variables.
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Block Design• Experiments are often blocked by– Age– Gender– Race– Achievement Level (Regular, Honors, AP, etc.)