Questions

1. Questions • What is the relationship between ‘research designs’ and ‘research strategies’? • Which method of experiments, within subjects or between subjects design is used more? • In the staged emergencies in field studies, are the people that are observed being deceived? • Are simulations most commonly used to test social norms? • Has it ever been the case that a study is tested with different designs? Is it effective to do this?

2. Questions • How do researchers deal with threats to internal validity, such as compensatory equalization? • Is it ethical to give a participant a placebo if the drug being tested is needed by the participant? • How soon can we get our grade for our presentations? Do you have a checklist for all of the parts we need to include?

3. Experimental Designs: Within-subjects design Chapter 9 Dusana Rybarova Psyc 290B May 25 2006

4. Outline: • Introduction – Characteristics of within-subjects designs • Advantages and disadvantages of within-subjects designs • Dealing with time-related threats and order effects • Applications and statistical analyses of within-subjects designs • Matched-subjects designs

5. 1. Introduction – Characteristics of within-subjects designs • a within subjects experimental design compares two or more different treatment conditions (or compares treatment and control) by observing or measuring the same group of individuals in all of the treatment conditions being compared • a within-subjects design looks for differences between treatment conditions within the same group of participants • a within subjects design is often called a repeated-measures design because the research study repeats measurements of the same individuals under different conditions • it is used in experimental situations comparing different treatment conditions and also to investigate changes occurring over time

6. 2. Advantages and disadvantages of within-subjects designs • Advantages of within-subjects designs • it requires relatively few participants • it essentially eliminated all of the problems based on individual differences that are the primary concern of a between-subjects designs • a within-subjects design has no differences between groups • each individual serves as his or her own control or baseline

7. 2. Advantages and disadvantages of within-subjects designs • Disadvantages • time-related problems • participant attrition • history • any outside events that occur during the time that a within-subjects experiment is being conducted and has an influence on the participants’ scores • maturation • any physiological or psychological changes that occur in a participant during the time a within-subjects experiment is conducted and that can influence the participants’ scores (e.g. young children)

8. 2. Advantages and disadvantages of within-subjects designs • Disadvantages • time-related problems (cont.) • instrumentation • refers to changes in the measurement instrument that occur over time (e.g. observer changes) • statistical regression • or regression toward the mean is a mathematical phenomenon in which extreme scores (high and low) on one measurement tend to be less extreme on a second measurement (especially a problem when participants are selected for their extreme scores)

9. 2. Advantages and disadvantages of within-subjects designs • Disadvantages • order effects • carryover effects • changes in behavior or performance that are caused by participation in an earlier treatment condition • carryover effects exist whenever one treatment condition produces a change in the participants that affects their scores in subsequent treatment conditions (e.g. new skill from treatment 1 can influence results in treatment 2) • progressive error • changes in participant's behavior or performance that are related to experience over time in a research study but not related to a specific treatment or treatments (e.g. practice effects and fatigue)

10. 3. Dealing with time-related threats and order effects • controlling time • if the different treatment conditions are scheduled over a period of weeks, the chances greatly increase that the results will be influenced by some outside event (history) or maturation or change in the measurement instrument • when a within-subjects design is not a good idea • e.g. comparing two methods of teaching reading to first-grade children (carryover effects)

11. 3. Dealing with time-related threats and order effects • counterbalancing • involves changing the order in which treatment conditions are administered from one participant to another • the goal is to use every possible order of treatment with an equal number of individuals participating in each sequence • the purpose of counterbalancing is to eliminate the potential for confounding by disrupting any systematic effects from factors related to time or the order of treatments • e.g. with two treatments one half of the participants begins in treatment 1, then moves to treatment 2 and the other half begins in treatment 2, then receives treatment1

12. 4. Applications and statistical analyses of within-subjects designs • two-treatment designs • a repeated-measures t test or an analysis of variance can be used to evaluate the statistical significance of the mean difference • if the data are measured on an ordinal scale, a Wilcoxon test can be used to evaluate significant differences • multiple-treatment designs • with too many treatment conditions, the distinction between treatments may become too small to generate significant differences in behavior • statistical analysis – repeated-measures analysis of variance to test for any significant differences among the treatment means

13. 5. Matched-subjects designs • each individual in one group is matched with a participant in each of the other groups • the matching is done so that the matched individuals are equivalent with respect to a variable that the researcher considers to be relevant to the study (e.g. IQ) • maintains all the advantages of between-subjects and within-subjects designs without the limitations of either (e.g. eliminates individual differences, time-related factors and order effects)