Chapter 9: Experimental Designs, Within-subjects (Research Methods)

Within-Subjects Experimental Design

An experimental design in which the same group of individuals participates in all of the different treatment conditions. Also know as repeated-measures experimental design.

History

A threat to internal validity from any outside event that influences the participants’ scores in one treatment differently than in another treatment.

Instrumentation

A threat to internal validity from changes in the measurement instrument that occur during the time a research study is being conducted. Also know as instrumental bias or instrumental decay.

Maturation

A threat to internal validity from any physiological or psychological changes that occur in a participant during the time that research study is being conducted and that can influence the participant’s scores.

Carry-over effects

Changes in the scores observed in one treatment condition that are caused by the lingering aftereffects of a specific earlier treatment condition.

Statistical regression

A statistical phenomenon in which extreme scores (high or low) on a first measurement tend to be less extreme on a second measurement; considered a threat to internal validity because changes in participants’ scores could be caused by regression rather than by the treatments.

Order effect

Whenever individuals participate in a series of treatment conditions and experience a series of measurements, their behavior or performance at any point in the series may be influenced by experience that occurred earlier in the sequence.

Progressive Error

In a research study, changes in the scores observed in one treatment condition that are related to general experience in a research study over time, but not to a specific treatment or treatments. Common kinds of progressive error are practice effects and fatigue.

Contrast effect

An example of a carryover effect in which the perception of a treatment condition is influenced by its contrast with the previous treatment.

Counterbalancing

In a within-subjects design, a procedure to minimize threats from order effects and time-related factors by 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.

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.

Participant Attrition

The loss of participants that occurs during the course of a research study conducted over time. Attrition can be a threat to internal validity. Also known as participant mortality.

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.

instrumentation or instrumental bias or instrumental decay

A threat to internal validity from changes in the measurement instrument that occur during the time a research study is being conducted. Also known as instrumental bias or instrumental decay.

within-subjects experimental design or repeated-measures experimental design

An experimental design in which the same group of individuals participates in all of the different treatment conditions. Also known as a repeated-measures experimental design.