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Pre and True Experimental Research Designs

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Title: Pre and True Experimental Research Designs


1
Chapter 11
  • Pre- and True Experimental Research Designs

2006 Prentice Hall, Salkind.
2
CHAPTER OVERVIEW
  • Experimental Designs
  • Internal and External Validity and Experimental
    Design
  • Controlling Extraneous Variables

3
TRUE EXPERIMENTAL RESEARCH METHODS
  • Allow statements about cause and effect
  • By controlling potential sources of variance
  • The simplest experimental design
  • Randomly selects subjects from population
  • Experimental groupreceives treatment
  • Control groupdoes not receive treatment

4
EXPERIMENTAL DESIGNS
5
ONE-SHOT CASE STUDY DESIGN
  • No randomization

6
ONE GROUP PRETEST POST-TEST DESIGN
  • Compare same subjects before and after treatment
  • No randomization
  • No control group
  • Little ability to infer cause and effect

7
TRUE EXPERIMENTAL DESIGNS
  • Characteristics
  • Random assignment
  • Control group
  • Three typical designs
  • Pretest post-test control group design
  • Post-test only control group design
  • Solomon four-group design

8
PRETEST POST-TEST CONTROL GROUP DESIGN
  • Groups should be equivalent at beginning
  • SO
  • Observed differences must result from treatment

9
POST-TEST ONLY CONTROL GROUP DESIGN
  • Use when
  • Sample is sufficient ( 30/group)
  • Pre-testing is not possible
  • Disadvantages
  • If randomization is not effective, groups may not
    be equivalent
  • Cannot use pretest to assign to groups

10
SOLOMON FOUR GROUP DESIGN
  • Many different comparisons are possible

11
INTERNAL AND EXTERNAL VALIDITY AND EXPERIMENTAL
DESIGN
  • Internal validityThe accuracy in concluding that
    the outcome of an experiment is due to the
    independent variable
  • External validityThe extent to which the results
    of an experiment can be generalized

12
THREATS TO INTERNAL VALIDITY
  • HistoryUncontrolled outside influences on
    participants during an experiment
  • MaturationChanges due to natural development
  • SelectionBiased selection of participants
  • TestingSensitization due to pretest
  • InstrumentationBiases due to testing procedures
  • RegressionThe tendency for extreme scorers to
    move toward more typical performance when
    retested
  • MortalityChanges in group composition because
    some participants have left the study

13
THREATS TO EXTERNAL VALIDITY
  • Multiple treatment interferenceSeveral
    treatments occur simultaneously
  • Reactive arrangements (Hawthorne
    effect)Knowledge about the experiment
  • Experimenter effectsEffects due to the presence
    of the experimenter
  • Pretest sensitizationSensitization due to pretest

14
INCREASING INTERNAL AND EXTERNAL VALIDITY
  • Increasing internal validity
  • Randomly select participants
  • Randomly assign to groups
  • Use a control group
  • Increasing external validity
  • Careful adherence to good experimental practices!

15
INTERNAL OR EXTERNAL VALIDITY A TRADE-OFF?
  • Too much control reduces ability to generalize
  • Too little control reduces ability to make causal
    statements
  • Attempt to find a good balance

16
CONTROLLING EXTRANEOUS VARIABLES
  • Variables that are not accounted for can confound
    an experiment
  • Controlling extraneous variables
  • Ignore them if they are unrelated to the
    dependent variable
  • Randomizing helps ensure that groups are
    equivalent

17
MATCHING
  • Ensures that subjects in each group
  • Are equivalent on some characteristic
  • Should be related to the dependent measure
  • Disadvantages
  • Expensive and time consuming
  • May not be possible
  • Matching on some variables establishes
    equivalence on others

18
USE OF HOMOGENEOUS GROUPS
  • Select sample from a population whose members are
    alike on critical factors

19
ANALYSIS OF COVARIANCE (ANCOVA)
  • A statistical tool that equalizes any initial
    differences that might exist
  • Between groups
  • On a covariate (a potential matched variable)
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