Transcript Chapter8

Experimental Research
Experiments
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Begin with a Hypothesis
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Modify Something in a Situation
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Compare Outcomes
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Cases or People are Termed “Subjects”
Random Assignment
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Probability of Equal Selection
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Allows Accurate Prediction
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An Alternative to Random Assignment
is Matching
Parts of the Classic Experiment
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Treatment or Independent Variable
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Dependent Variable
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Pretest
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Posttest
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Experimental Group
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Control Group
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Random Assignment
Variations on Experimental Design
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Pre-experimental Design
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One-shot Case Study
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One-group Pretest-Posttest Design
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Static Group Comparison
Quasi-Experimental and Special Designs
Types of Validity
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External Validity
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Do the results apply to the broader population?
Internal Validity
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Is the independent variable responsible for the
observed changes in the dependent variable?
Confounding Variables That Threaten
Internal Validity
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Maturation
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Changes due to normal growth or predictable
changes
History
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Changes due to an event that occurs during
the study, which might have affects the results
Confounding Variables That Threaten
Internal Validity
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Instrumentation
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Regression to the Mean
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Any change in the calibration of the measuring instrument
over the course of the study
Tendency for participants selected because of extreme
scores to be less extreme on a retest
Selection
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Any factor that creates groups that are not equal at the
start of the study
Confounding Variables That Threaten
Internal Validity
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Attrition
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Loss of participants during a study; are the
participants who drop out different from those
who continue?
Diffusion of treatment
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Changes in participants” behavior in one
condition because of information they obtained
about the procedures in other conditions
Subject Effects
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Participants are not passive
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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
Experimenter Effects
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Any preconceived idea of the researcher
about how the experiment should turn
out
Compensatory effects
Types of Control Procedures
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General control procedures
virtually all research)
(applicable
to
Control over subject and experimenter effects
Control through the selection and assignment
of participants
Control through specific experimental design
Principles of Experimental Design
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Control the effects of lurking
variables on the response, most
simply by comparing two or more
treatments
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Randomize
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Replicate
Randomization
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The use of chance to divide experimental
units into groups is called randomization.
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Comparison of effects of several treatments is
valid only when all treatments are applied to
similar groups of experimental units.
How to randomize?
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Flip a coin or draw numbers out of a hat
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Use a random number table
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Use a statistical software package or program
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Minitab
www.whfreeman.com/ips
Statistical Significance
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An observed effect so large that it would
rarely occur by chance is called
statistically significant.
A few more things…
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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?