In between-subjects experimental designs, we randomly assign different subjects to each of the levels of the independent variable. That is, for an experiment with one IV with two levels or conditions, half of the subjects are exposed to the first level of the independent variable and the other half of subjects are exposed to the second level of the independent variable. For each participant, his/her score on the dependent variable is collected following exposure to the independent variable. Dr. Z's examination of the effects of type of course material on grades in the course is a between-subjects design, in that, one set of students were assigned to receive traditional course materials while a second set of students were assigned to receive the traditional materials accompanied by interactive tutorials. In other words, subjects could only belong to one level of the independent variable.
In within-subjects experimental designs, each subject in the study is exposed to each level of the independent variable. Therefore, for each subject a score on the dependent variable is collected more than once (once for each level of the independent variable). At times you may hear this design referred to as a repeated-measures design, since all subjects are repeatedly measured on the dependent measure for each level of the independent variable. For example, suppose that you hypothesized that you could alleviate the fear of public speaking by training people to engage in some deep breathing before beginning their speech. For the control condition (absence of treatment) you have a number of participants give a short speech introducing themselves to a small crowd of on-lookers. Immediately following the speech, you measure the participants' heart rates as a measure of the stress or fear they have experienced. A week later, you expose the same participants to the treatment condition. You give these same participants some training in deep breathing exercises and instruct them to use this technique before speaking in public. Once again, you ask these same participants to give a short speech introducing themselves to a small crowd and, once again, you take the participants' heart rate immediately following the speech. This experiment uses a within-subjects design, in that all participants in the study were exposed to each level of the IV (control condition and deep breathing condition).
To illustrate the differences between within-subjects and between-subjects designs, suppose we wanted to test the effects of eating sweets on children's attention. We might have two levels of our independent variable: exposed to no sweets in the hour preceding the attention test, and fed a 50g chocolate bar in the hour preceding the attention test. If we had 8 children available to us to participate in the study (this is a hypothetical situation, typically we would require higher sample sizes), then here is an illustration of how the children may be assigned to the groups or conditions of the independent variable and when the dependent variable may be measured.
No Sweets Condition (Control Group): Collect attention scores for Joe, Jim, Judy, & Jill
Chocolate Bar Condition (Experimental Group): Collection attention scores for Kyle, Kurt, Kathy, & Kali
No Sweets Condition (Control Condition): Collect attention scores for Joe, Jim, Judy, Jill, Kyle, Kurt, Kathy, & Kali
Chocolate Bar Condition (Experimental Condition): On the following day, collect attention scores for Joe, Jim, Judy, Jill, Kyle, Kurt, Kathy, & Kali