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Psychology 2001 Chapter 9 – Within subjects design

Characteristics of a within subjects design * The defining characteristic of a within subjects design is that it uses a single group of participants, and tests or observes each individual in all of the different treatments being compared. * Often called a repeated measures design because the research study repeats measurements of the same individuals under different conditions

Advantages of a within subjects design * it requires relatively few participants * Essentially eliminates all of the problems based on individual differences that are the primary concern of a between subjects design. * It is possible to measure the differences between subjects without involving any individual differences * It is possible to measure the differences between individuals. When the individual differences are consistent across treatments, they can be measured and removed from the rest of the variance in the data.

Disadvantages of a within subjects design * The primary disadvantage comes from the fact that each participant usually goes through a series of treatment conditions with each treatment administered at a different time * Opportunity for time related factors * PARTICIPANT ATTRITION: some of the individuals who start the research study may be gone before the study is completed – this can exaggerate volunteer bias if only the most committed volunteers remain in the study.

Threats to internal validity * Environmental variables: Characteristics of the environment that may change from one treatment to another. * Confounding of environmental factors: During the time between the first measurement and the final measurement, the participants may be influenced by a variety of factors other than the treatments being investigated.
- History: Scores may be affected by changing events outside the study
- Maturation: Scores may be affected by physiological or psychological changes in the participants
- Instrumentation: Scores may be affected by changes in the measuring instrument
- Testing effects: Scores may be affected by experience in prior treatment conditions
- Statistical regression: Extreme scores may become less extreme as a result of statistical regression

Separating Time related factors and order effects * Testing effects are directly related to experience obtained by participating in previous treatment conditions. * Order effects are when participants go through a series of treatments in order, and that performance in any treatment may be influenced by previous treatments * Carry over effect: when an order effect is caused by a specific previous treatment. * An example of a carry over effect is a Contrast effect: In which the perception of one treatment condition is influenced by its contrast with the previous treatment. * Progressive Error: is when order effects aren’t linked to a specific treatment, but linked to general experience accumulating during the study.
- Practice effects: Improvement as a result of previous testing
- Fatigue effects: Progressive decline in performance.

Dealing with time-related threats and order effects. * Controlling time
- Although shortening the time between

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