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Experimental Designs

Experimental Designs. Single IV Designs. The basic two-group design. Independent Variable. Condition 1. Condition 2. Single IV Designs. Independent groups Randomly assigned to groups Kasser and Sheldon (2000). Mortality salience. Music. Single IV Designs. Nonequivalent groups

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Experimental Designs

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  1. Experimental Designs

  2. Single IV Designs • The basic two-group design Independent Variable Condition 1 Condition 2

  3. Single IV Designs • Independent groups • Randomly assigned to groups Kasser and Sheldon (2000) Mortality salience Music

  4. Single IV Designs • Nonequivalent groups • IV is a subject variable • Reduce nonequivalence as much as possible by matching on important dimensions McDonald & Flanagan (2004) Non-TBI TBI

  5. Single IV Designs • Correlated groups • Matched groups • Natural pairs Blagrove (1996) Sleep-deprived Non-sleep-deprived

  6. Single IV Designs • Within-subject design • Repeated measures Lee and Aronson (1974) Moving forward Moving backward

  7. Single IV Designs • Advantages of independent-groups designs • Simplicity • In some contexts, it is impossible to use correlated groups

  8. Single IV Designs • Advantages of independent-groups designs • Simplicity • In some contexts, it is impossible to use correlated groups • Advantages of correlated-groups designs • Control—we have greater certainty of equality • Statistical benefits

  9. Statistics in a Two-Groups Design • Conceptually, what are we testing in a two-groups design? • What’s the null hypothesis? • What’s the alternative hypothesis? • What test should be used?

  10. between-groups variability statistic = error variability Statistics in a Two-Groups Design • Two sources of variability in your data: • The IV, or between-groups variability • Error variability, or within-groups variability

  11. Observations in a study can be divided into two components: • Signal: The key variable—the construct you’re trying to measure • Noise: All random factors in the situation that make it harder to see the signal

  12. Observation Signal Noise

  13. Signal Noise • You want the signal to be high relative to the noise

  14. Signal between-groups variability statistic = error variability Noise Statistics in a Two-Groups Design • Two sources of variability in your data: • The IV, or between-groups variability • Error variability, or within-groups variability

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