1. Questions  Is Exam 2 going to be cumulative or will it just cover the second part of the information?  Are cause-and-effect relationships the same as causal relationships?  Can you give a clear example of the difference between confounding variables and extraneous variables? –Extraneous variables – any variables other than the studied dependent and independent variables in a study (e.g. random time of day) –Confounding variables – extraneous variables that change systematically with the studied variables (e.g. time of day systematically varied with a treatment)  Do researchers need to address extraneous variables in their study, or only when the extraneous variables become confounding variables that effect the dependent variable?  Can you have more than one dependent variable?

2. Questions  Are errors in research thought of as being a third- variable (such as environmental or participant changes)? I guess I don’t really understand the difference between a third-variable, errors, and extraneous variables. –Third-variable is a confounding variable and a confounding variable is a kind of extraneous variable  Can manipulation be deceptive… If so can the manipulation be a problem when it domes to ethics?  Are we skipping chapter 7 because 7 was on the syllabus but today we did chapter 8.

3. Experimental Designs: Between- subjects design Chapter 8 Dusana Rybarova Psyc 290B May 24 2006

4. Outline: 1.Introduction – Characteristics of between- subject design 2.Advantages and disadvantages of between-subjects designs 3.Within and between treatments variability 4.Other threats to internal validity of between-subjects designs 5.Applications and statistical analyses of between-subjects designs

5. 1. Introduction – Characteristics of between-subject design  There are two basic research designs associated with the experimental research strategy –between-subjects design  we obtain each of the different groups of scores from a separate group of participants  e.g. one group of students is assigned to teaching method A and a separate group to method B –within-subjects design  different groups of scores are all obtained from the same sample of participants  e.g. one sample of individuals is given a memory test using a list of one-syllable words, and then the same set of individuals is tested again using a list of two-syllable words

6. 1. Introduction – Characteristics of between-subject design  the defining characteristic of a between-subjects design is that it compares separate groups of individuals  another feature of a between-subjects design is that it allows only one score per participant (every score represents a separate, unique participant)  because each score represents a separate participant, a between subjects design is often called an independent-measures design

7. 1. Introduction – Characteristics of between-subject design  a between-subjects experimental design requires a separate, independent group of individuals for each treatment condition compared  individuals are assigned to groups using a procedure that attempts to create equivalent groups  the general goal of between-subjects experiment is to determine whether differences exist between two or more treatment conditions (e.g. a researcher may want to compare two teaching methods (two treatments) to determine whether one is more effective than the other)

8. 2. Advantages and disadvantages of between-subjects designs  Advantages –each individual score is independent of the other scores –participant’s score is not influenced by such factors as:  practice or experience gained in other treatments  fatigue or boredom from participating in a series of treatments  contrast effects that result from comparing one treatment to another (e.g. room temperature)

9. 2. Advantages and disadvantages of between-subjects designs  Disadvantages –large number of participants (problem with special populations) –individual differences  characteristics that differ from one participant to another are called individual differences  individual differences can become confounding variables  individual differences can produce high variability in the scores

10. 2. Advantages and disadvantages of between-subjects designs  Confounding variables in between subjects designs –individual differences  participant characteristics differ from one group to another  e.g. the participants in one group may be older, smarter, taller etc. than the participants in another group –environmental variables  characteristics of the environment differ between groups  e.g. one group may be tested in a large room and another group in a smaller room

11. 2. Advantages and disadvantages of between-subjects designs  Equivalent groups –in a between-subjects experimental design, the researcher does have control over the assignment of individuals to groups –the separate groups must be:  created equally  treated equally (except for the treatment conditions)  composed of equivalent individuals

12. 2. Advantages and disadvantages of between-subjects designs  Limiting confounding by individual differences –random assignment (randomization)  a random process is used to assign participants to groups –matching groups (matched assignment)  involves assigning individuals to groups so that a specific variable is balanced or matched across the groups (e.g. IQ) –holding variables constant  simply hold the variable constant (e.g. restrict the participants to those with IQs between 100-110)

13. 3. Within and between treatments variability  advantage –variability between treatments –it can be increased by increasing differences between conditions (levels)  disadvantage –variability within treatments –it is caused by individual differences –should be minimized

14. 3. Within and between treatments variability  minimizing variability within treatments –standardize procedures and treatment setting –limit individual differences by holding a participant variable constant –random assignment and matching –sample size  using a large sample can help minimize the problems associated with high variability

15. 4. Other threats to internal validity of between-subjects designs  assignment bias –groups of participants are different before the treatments –the group assignment process produces groups with noticeably different characteristics  differential attrition –attrition refers to participant withdrawal from a research study before it is completed –differential attrition refers to differences in attrition rates from one group to another and can threaten the internal validity of a between-subjects experiment (e.g. effectiveness of a dieting program)

16. 4. Other threats to internal validity of between-subjects designs  diffusion or imitation of treatment –refers to the spread of the treatment effects from the experimental group to the control group (e.g. new depression therapy)  compensatory equalization –occurs when an untreated group learns about the treatment being received by another group and demands the same or equal treatment (e.g. watching Batman in violent TV group)

17. 4. Other threats to internal validity of between-subjects designs  compensatory rivalry –occurs when an untreated group learns about the treatment received by another group and then works extra hard to show that they can perform just as well as the individuals receiving the special treatment  resentful demoralization –opposite of compensatory rivalry –occurs when an untreated group learns about the treatment received by another group and is less productive and less motivated because they resent the expected superiority of the treated group

18. 5. Applications and statistical analyses of between-subjects designs  comparing only two groups of participants –this design is referred to as the single-factor two-group design or simply two group design –an independent-measures t test is used to determine whether there is a significant difference between the means  comparing means for more than two groups –e.g. single factor multiple group design may be used and analysis of variance (ANOVA) would be used for statistical analysis –adding extra groups to a research study tends to reduce the differences between groups