Section 5.2 Designing Experiments

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Section 5.2Designing Experiments

• AP Statistics
• www.toddfadoir.com/apstats

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Terminology

• The individuals on which the experiment is done
  are the experimental units.
• When the units are human beings, they are called
  subjects.
• A specific experimental condition applied to the
  units is called the treatment.

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Experiments
Units
Treatment
Observe Response
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More terminology

• The explanatory variables in an experiment are
  often called factors.
• Each treatment is formed by combining a specific
  value (often called a level) of each of the
  factors.

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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.

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The Physicians Health Study

• Subjects?
• Physicians
• Treatments?
• 4 (the groups-gt
• Factors?
• 2 (aspirin beta carotene)

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The Placebo Effect

• Gastric freezing is a clever treatment for
  ulcers in the upper intestine. The patient
  swallow a deflated balloon with tubes attached,
  the 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.
• An experiment reported in the Journal of the
  American Medical Association showed that gastric
  freezing did reduce the acid production of and
  relieve ulcer pain.

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The Placebo Effect

• The Gastric freezing experiment was poorly
  designed. The patients response may have been
  due the placebo effect. A placebo is a dummy
  treatment. Many patients respond favorably to any
  treatment, even a placebo.
• This may be due to trust in the doctor and
  expectations of a cure, or simply to the fact
  that medical conditions often improve without
  treatment.

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Placebo Use

• 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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Experiments
Compare Response
Units
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Principles of Experiment Design

• Control the effects of lurking variables on the
  response, most simply 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.

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Statistical Significance

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

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Double-Blind

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

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Experiments without placebos

• Matched pair design
• In a 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.

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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.

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Assignment

• Exercises 5.31-5.57 odd