Final answer:
In a counterbalanced within groups design, presence or absence of sunglasses would be varied by alternating their use across different driving sessions for each participant.
Step-by-step explanation:
In a counterbalanced within-groups design, researchers systematically vary the order of conditions to mitigate potential biases. Here, participants would experience both driving with and without sunglasses, but in a randomized order. For instance, half the participants might drive without sunglasses first, while the other half starts with sunglasses. After an interval, they switch conditions. This approach helps account for individual differences and external factors that might affect driving performance. By comparing each participant's performance across both conditions, researchers can better isolate the impact of sunglasses on driving ability. Counterbalancing controls for the order effect, where performance could be influenced by simply doing the task first or second. It ensures that any differences observed in driving performance are more likely due to the presence or absence of sunglasses.
In a counterbalanced within-groups design, the presence or absence of sunglasses would be alternated to explore their impact on driving performance.