Final answer:
In a within-subjects design, group differences are not a concern since the same participants are exposed to all conditions, making 'd) Group differences' the correct answer since it would only be relevant in a between-subjects design.
Step-by-step explanation:
In a within-subjects design, all participants are exposed to every treatment or condition. The primary advantage of this design is that individual differences between participants are controlled for, as the same individuals participate in all conditions. This means that the differences between experimental conditions could be attributed to order effects, where the sequence of presenting the treatments affects the results; practice effects, which involve participants improving in a task simply because they have done it more than once; or other factors that affect the outcome, such as fatigue or boredom. However, group differences are not a concern in within-subjects designs because there are no separate groups; instead, the same group of individuals experience all conditions.
Therefore, the correct answer is d) Group differences, as this factor would only be relevant in a between-subjects design where different individuals are assigned to each condition or treatment group.