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Experimental Designs

Experimental Designs. Meet all the following criteria: Test one or more hypotheses about causal effects of the independent variable (IV) Include at least two levels of the IV Randomly assign participants to conditions Provide for: Specific procedures for testing hypotheses

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Experimental Designs

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  1. Experimental Designs • Meet all the following criteria: • Test one or more hypotheses about causal effects of the independent variable (IV) • Include at least two levels of the IV • Randomly assign participants to conditions • Provide for: • Specific procedures for testing hypotheses • Control for the major threats to internal validity Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  2. Variance in Experiments • Research designs • Control unwanted sources of variance to allow us to evaluate the effects of the independent variable on the dependent variable • Systematic between-groups variance • Experimental variance (due to the IV) • Extraneous variance (due to confounding variables) • Nonsystematic within-groups error variance • Due to chance factors and individual differences Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  3. Controlling Variance • Maximizing experimental variance • Make sure that there are real differences between the groups • Controlling extraneous variance • Make sure the groups are as similar as possible at the start of the study • Minimizing error variance • Control with careful measurement or with special designs (e.g., correlated-group designs) Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  4. Between-Group Designs • Independent groups or conditions • The participants serve in only 1 group or condition • Randomly assigned to an experimental condition or a control condition • Independent means or averages • The means from each group are independent as they come from non-related participants • Use specific analysis to test these group mean differences. Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  5. Within-Group Designs • Non-independent groups or conditions • Same participants in each group • All participants are exposed to all experimental conditions • Non-independent means • The group means are related as they come from the same participants • Use a different test of these means than is used in the between group design. Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  6. Within-Group Design Strengths • More sensitive to small group differences • variability due to individual differences is statistically eliminated • Fewer participants are needed • each participant appears in each condition Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  7. Within-Group Design Weaknesses • Because participants experience all conditions, they may figure out the hypothesis • potential subject bias effects • Sequence effects • Practice and carry-over effects from the first condition • Controlled by varying the order of presentation • Counterbalancing • Random order of presentation • Latin square design Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  8. Mixed Designs • Many designs are a combination of between and within group comparisons. • Example: Studying the effects of stress on heart rate for women and men before and after relaxation training. • Comparing the means from the men and women on the DV is a between group test • Comparing the means for each gender before and after the relaxation training is a within group test. Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  9. Statistical Analysis • Between Groups Designs • Independent t-test • ANOVA • MANOVA • For designs with more than one IV • Within Groups Designs • Correlated or paired groups t-test • Repeated measures ANOVA Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

  10. Summary • Research is designed to measure and control sources of variance • There are a variety of non-experimental and experimental designs available • Experimental designs have two elements • Random assignment of participants to conditions • A control group Dr.Launius Psy 1191 Adapted from Graziano & Raulin (2004)

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