Chapter 8 Lecture

# Chapter 8 Lecture - Chapter 8 Experimental Design Copyright...

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Copyright Atomic Dog Publishing, 2005 Chapter 8 Experimental Design

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Copyright Atomic Dog Publishing, 2005 Experimental Designs and Internal Validity Strongest of all designs regarding internal validity Also intrusive and difficult to carry out in the “real world” Trade off to high internal validity may be reduced external validity (generalizability) due to “contrived” conditions of the experiment Three conditions Random assignment to groups Control group Manipulation of the independent variable
Copyright Atomic Dog Publishing, 2005 Two-Group Experimental Designs Note that a pretest is not necessary in this design. Why? Because random assignment (R) assures that we have probabilistic equivalence of groups Strongest design with respect to threats to internal validity R X O R O The Basic Design Two group, post-test only, randomized experiment

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Copyright Atomic Dog Publishing, 2005 Two-Group Experimental Designs Difference between groups on posttest (O) indicates a treatment effect Usually test this with a t-test or one-way ANOVA R X O R O The Basic Design Two group, post-test only, randomized experiment
Copyright Atomic Dog Publishing, 2005 Threats to Internal Validity for the Posttest-Only Randomized Experimental Design

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Copyright Atomic Dog Publishing, 2005 Probabilistic Equivalence It means that we know perfectly the odds that we will find a difference between the two groups. It doesn’t mean that the two groups will have identical means.
Copyright Atomic Dog Publishing, 2005 Probabilistic Equivalence

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Copyright Atomic Dog Publishing, 2005 Random Selection and Random Assignment Random selection is how you draw the sample of people for your study from a population—impacts external validity. Helps insure that the sample is representative of the population (and hence, findings are more generalizable ) Random assignment is how you assign the sample to different groups or treatments in your study—impacts internal validity . Helps insure that groups are comparable at the beginning of the study
Copyright Atomic Dog Publishing, 2005 Classifying Experimental Designs “Signal to noise ratio” metaphor “Signal enhancer” designs— factorial designs “Noise reducer” designs— blocking and covariance designs

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Copyright Atomic Dog Publishing, 2005 Observed Time Series
Copyright Atomic Dog Publishing, 2005 Experimental Designs Factorial Designs Terminology: Factor

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## This note was uploaded on 02/11/2011 for the course CHFD 5110 taught by Professor Johnson during the Spring '11 term at University of Georgia Athens.

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Chapter 8 Lecture - Chapter 8 Experimental Design Copyright...

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