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Quasi Experimental Designs

Quasi Experimental Designs

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Quasi Experimental Designs

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  1. Quasi Experimental Designs An Alternative to the True Experiment

  2. Characteristics ofQuasi-experimental designs • Goals and consequences • Basic versus applied research • Experimental Control • Have manipulation of the independent variable • May lack either or both of: • Random assignment to groups • Control group • External validity • Potential for bias during data collection • Leading participants to desired answer • Selective recording of data

  3. Advantages of Quasi-Experimental Designs • Provide designs for studying those situations in which random assignment is not possible while only minimally compromising the internal validity of the study

  4. Disadvantages of Quasi-Experimental designs • Increased potential for bias associated with sampling • Threats to internal validity • See Maas, Buckwalter, Reed, and Specht (1998); Cook and Campbell (1979); Campbell and Stanley (1966)

  5. Threats to Internal Validity • History • Maturation • Testing • Instrumentation • Regression to the Mean • Subject Mortality • Selection

  6. Additive Effect of Selection and Maturation Pretest Posttest

  7. Additive Effect of Selection and History • The Local History Effect – one group of third graders may have gotten new books, or may have gone on a special field trip that really inspired them to get more involved in learning.

  8. Other Possible Threats to Internal Validity (even true experiments may not eliminate • Experimenter expectancy effects • Contamination • Hawthorne effects

  9. Types of quasi-experimental designs • Nonequivalent control group designs O1X O2 - - - - - - - - - - - - - - - O1 O2 • Interrupted time-series designs O1 O2 O3X O4 O5 O6 O1X O2X O3X O4

  10. Types of quasi-experimental designs • Time series with nonequivalent control group designs O1 O2 O3X O4 O5 O6 - - - - - - - - - - - - - - - - - - - - - - - - O1 O2 O3 O4 O5 O6

  11. Problem • A local business person has asked for your help. They want to develop special shoes to improve happiness. They have a product ready for testing. Once again, money is endless. Design a quasi-experimental study to answer this question.

  12. What about external validity? • How do we know if these findings have external validity?