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Experimental Designs: Between-Subjects Design I. Experimental Research Strategy A. Manipulation of IV B. Control of extraneous variables C. Basic Experimental Designs 1. between-subjects design--Participants serve in only one treatment condition. 2. within-subjects design--Participants serve in more than one treatment condition. D. Structure of Between-Subjects Designs  A separate group of participants is assigned to each of the different conditions by using random selection. Groups are presumed to be equal at start so that we can conclude any difference in DV is due to effect of IV. In order to assure equivalent groups, the groups must be created equally, treated equally, and composed of equivalent individuals. E. Advantages & Disadvantages of Between-Subjects Designs Pro: Each participant is exposed to only one treatment condition. Therefore, score is free from the following influences: (1) practice/experience, (2) fatigue/boredom, and (3) contrast effects. Also, usable for wide variety of research questions. Cons: (1) Need relatively large N and (2)each score is obtained from a different person--all possessing different characteristics. Individual Differences--e.g., gender, age, personality, background Caution: Individual differences can become confounding variables and can produce high variability in the scores. II. Limiting Confounding by Individual Differences Random Assignment (randomization) Matching Groups (matched assignment) Holding Variables Constant III. Variability and Individual Differences Figure 10.3 Results From a Simulated Experiment Comparing the Conditions Using Participants Selected From a Population Where Individual Differences are Relatively Small (p. 253) Figure 10.4 Results from a Simulated Experiment Comparing Two Conditions using Participants Selected from a Population Where Individual Differences are Relatively Large (p. 254) Differences between treatments and variability within treatments Minimizing variability within treatments Standardize Procedures and Treatment Setting Limit Individual Differences Random Assignment and Matching Sample Size IV. Other Threats to Internal Validity of Between-Subjects Designs Assignment Bias Differential Attrition Diffusion or Imitation of Treatment Compensatory Equalization Compensatory Rivalry Resentful Demoralization Engagement Activities Ch. 8 1) A researcher has a sample of 30 rats that are all cloned from the same source. The 30 rats are genetically identical and have been raised in exactly the same environment since birth. The researcher conducts an experiment, randomly assigning the remaining 10 clones to Treatment A, 10 to Treatment B, and the other 10 to Treatment C. Explain why the clone experiment is better than a between-subjects study using 30 regular rats that are randomly assigned to the three conditions. In other words, explain how the clone experiment eliminates the basic problems with a between-subjects study. 2) A recent survey at a major corporation found that employees who regularly participated in the company fitness program tended to have fewer sick days than employees who did not participate. However, because the study was not a true experiment, you cannot conclude that regular exercise causes employees to have fewer sick days. a) Identify another factor (a confounding variable) that might explain why some employees participated in the fitness program, and why those same employees have fewer sick days. b) Describe the design for a between-subjects experiment that would determine whether participating in the exercise program caused fewer sick days. c) Describe how the factor you identified in Part a is controlled in your experiment. What is a time related threat to internal validity for a within subjects experiment?Correct Answer : The major time-related factors that threaten within-subjects designs are history, instrumentation, maturation, and regression toward the mean. In each case, an outside factor can cause the participants' scores to change from one treatment to the next.
Which of the following threatens the internal validity of an experiment?What are threats to internal validity? There are eight threats to internal validity: history, maturation, instrumentation, testing, selection bias, regression to the mean, social interaction and attrition.
How might individual differences in between subjects designs threaten the internal validity of a study?Individual differences may threaten validity
Because different participants provide data for each condition, it's possible that the groups differ in important ways between conditions, and these differences can be alternative explanations for the results.
What can help rule out the threats to internal validity in an experimental design?Randomly assign participants to sample groups to eliminate regression and selection bias. This will make the groups comparable before you start your test. Another way to effectively eliminate threats to internal validity in your study is to make your participants blind to the purpose of your research.
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Differential attrition can threaten the internal validity of a between-subjects experiment.
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