Experimental Design

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Experimental Design
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Criteria for Establishing a Causal Relationship

• Concomitant variation
• Temporal variation
• Control over other possible causal factors

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Concomitant Variation
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Concomitant Variation
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Concomitant Variation
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Temporal Ordering

• Causes must precede effects.

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Control over other possible causal factors

• Randomization Randomly assign respondents to
  treatment conditions.
• Physical control Hold possible extraneous
  factors constant.
• Design control Experimental design.
• Statistical control Analysis of covariance.

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Threats to Validity

• Internal to the experiment
• External -- The relation of the experiment to
  real life

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Internal Threats

• History Events occurring simultaneously with
  the experiment
• Maturation Biological and/or psychological
  changes in the respondents
• Testing Measuring changes the objects measured.
• Instrumentation The yardstick can change.
• Selection Bias Improper assignment
• Mortality Differential loss of respondents

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External Threats

• Reactive/Interactive Effects of Testing
  Sensitizing respondents to the topic
• Interactive Effects of Selection Bias Improper
  assignment to groups
• Surrogate Situations The laboratory is not like
  the real world.

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