Experimental Research

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Experimental Research
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Experiments

• Begin with a Hypothesis
• Modify Something in a Situation
• Compare Outcomes
• Cases or People are Termed Subjects

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Random Assignment

• Probability of Equal Selection
• Allows Accurate Prediction
• An Alternative to Random Assignment is Matching

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Parts of the Classic Experiment

• Treatment or Independent Variable
• Dependent Variable
• Pretest
• Posttest
• Experimental Group
• Control Group
• Random Assignment

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Variations on Experimental Design

• Pre-experimental Design
• One-shot Case Study
• One-group Pretest-Posttest Design
• Static Group Comparison
• Quasi-Experimental and Special Designs

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Types of Validity

• External Validity
• Do the results apply to the broader population?
• Internal Validity
• Is the independent variable responsible for the
  observed changes in the dependent variable?

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Confounding Variables That Threaten Internal
Validity

• Maturation
• Changes due to normal growth or predictable
  changes
• History
• Changes due to an event that occurs during the
  study, which might have affects the results

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Confounding Variables That Threaten Internal
Validity

• Instrumentation
• Any change in the calibration of the measuring
  instrument over the course of the study
• Regression to the Mean
• Tendency for participants selected because of
  extreme scores to be less extreme on a retest
• Selection
• Any factor that creates groups that are not equal
  at the start of the study

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Confounding Variables That Threaten Internal
Validity

• Attrition
• Loss of participants during a study are the
  participants who drop out different from those
  who continue?
• Diffusion of treatment
• Changes in participants behavior in one
  condition because of information they obtained
  about the procedures in other conditions

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Subject Effects

• Participants are not passive
• They try to understand the study to help them to
  know what they should do
• This behavior termed subject effects
• Participants respond to subtle cues about what is
  expected (termed demand characteristics)
• Placebo effect treatment effect that is due to
  expectations that the treatment will work

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Experimenter Effects

• Any preconceived idea of the researcher about how
  the experiment should turn out
• Compensatory effects

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Types of Control Procedures

• General control procedures (applicable to
  virtually all research)
• Control over subject and experimenter effects
• Control through the selection and assignment of
  participants
• Control through specific experimental design

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Principles of Experimental Design

• Control the effects of lurking variables on the
  response, most simply by comparing two or more
  treatments
• Randomize
• Replicate

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Randomization

• The use of chance to divide experimental units
  into groups is called randomization.
• Comparison of effects of several treatments is
  valid only when all treatments are applied to
  similar groups of experimental units.

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How to randomize?

• Flip a coin or draw numbers out of a hat
• Use a random number table
• Use a statistical software package or program
• Minitab
• www.whfreeman.com/ips

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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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A few more things

• Double-blind neither the subjects nor the person
  administering the treatment knew which treatment
  any subject had received
• Lack of realism is a major weakness of
  experiments. Is it possible to duplicate the
  conditions that we want?