Chapter 9- Experimental Designs: Within-Subjects Designs Flashcards
Within-Subjects Experimental Design, OR, Repeated Measures Experimental Design
compares two or more different treatment conditions (or compares a treatment and a control) by observing or measuring the same group of individuals in all of the treatment conditions being compared. Thus, a within-subjects design looks for differences between treatment conditions within the same group of participants. To qualify as an experiment, the design must satisfy all other requirements of the experimental research strategy, such as manipulation of an independent variable and control of extraneous variables.
Threats to Internal Validity of Within-Subjects Experiments:
Confounding from Environmental Variables
characteristics of the environment that may change from one treatment condition to another. For example, one treatment may be evaluated during the morning and another treatment during the afternoon. Any such variable may cause differences in scores from one treatment to another and, therefore, provides an alternative explanation for the differences between treatments.
Threats to Internal Validity of Within-Subjects Experiments:
Confounding From Time-related Variables
A serious concern of within-subjects designs comes from the fact that the design often requires a series of measurements made over time. 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, and these other factors may affect the participants’ scores. If this occurs, then the internal validity of the study is threatened because a change in a participant’s score from one treatment to the next could be caused by an outside factor instead of the different treatments
Threat to internal validity: history
a threat to internal validity because any differences that are observed between treatment conditions may be caused by history instead of the treatments. When a group of individuals is being tested in a series of treatment conditions, any outside event(s) that influences the participants’ scores in one treatment differently than in another treatment is called a history effect.
threat to internal validity: instrumentation
refers to changes in the measuring instrument that occur during a research study in which participants are measured in a series of treatment conditions. Instrumentation is a threat to internal validity because any observed differences between treatment conditions may be caused by changes in the measuring instrument instead of the treatments.
threats to internal validity: maturation
when a group of individuals is being tested in a series of treatment conditions, any physiological or psychological change that occurs in participants during the study and influences the participants’ scores. Maturation is a threat to internal validity because observed differences between treatment conditions may be caused by maturation instead of the treatments.
threats to internal validity: Statistical Regression, or Regression Toward the Mean
a mathematical phenomenon in which extreme scores (high or low) on one measurement tend to be less extreme on a second measurement. Regression is a threat to internal validity because changes that occur in participants’ scores from one treatment to the next can be caused by regression instead of the treatments
threats to internal validity: order effects
occur when the experience of being tested in one treatment condition (participating and being measured) has an influence on the participants’ scores in a later treatment condition(s). Order effects threaten internal validity because any observed differences between treatment conditions may be caused by order effects rather than the treatments.
threats to internal validity: carry-over effects
Occur when one treatment condition produces a change in the participants that affects their scores in subsequent treatment conditions.
threats to internal validity: progressive error
refers to changes in a participant’s behaviour or performance that are related to general experience in a research study but not related to a specific treatment or treatments. Common examples of progressive error are practice effects and fatigue.
counterbalancing
For a within-subjects design, counterbalancing is defined as changing the order in which treatment conditions are administered from one participant to another so that the treatment conditions are matched with respect to time. The goal is to use every possible order of treatments 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 relationship between the order of treatments and time-related factors.
complete counterbalancing
In within-subjects designs, using a separate group of participants for every possible order of the treatment conditions. With n different treatment conditions, there are n! (n factorial) different orders.
partial counterbalancing
A system of counterbalancing that ensures that each treatment condition occurs first for one group of participants, second for one group, third for one group, and so on, but does not require that every possible order of treatment conditions be used.
latin square
An n × n matrix in which each of n different items appears exactly once in each column and exactly once in each row. Used to identify sequences of treatment conditions for partial counterbalancing
matched-subjects design
A research design comparing separate groups of individuals in which 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.
