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Experimental Research

Experimental Research. Experiments. Begin with a Hypothesis Modify Something in a Situation Compare Outcomes Cases or People are Termed “Subjects”. Random Assignment. Probability of Equal Selection Allows Accurate Prediction An Alternative to Random Assignment is Matching.

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Experimental Research

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  1. Experimental Research

  2. Experiments • Begin with a Hypothesis • Modify Something in a Situation • Compare Outcomes • Cases or People are Termed “Subjects”

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

  4. Parts of the Classic Experiment • Treatment or Independent Variable • Dependent Variable • Pretest • Posttest • Experimental Group • Control Group • Random Assignment

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

  6. 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?

  7. 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

  8. 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

  9. 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

  10. 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

  11. Experimenter Effects • Any preconceived idea of the researcher about how the experiment should turn out • Compensatory effects

  12. 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

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

  14. 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.

  15. 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

  16. Statistical Significance • An observed effect so large that it would rarely occur by chance is called statistically significant.

  17. 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?

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