Questions: Sarah investigated the effects of drinking orange juice on vision improvement. One group drank orange juice and the other drank water. Afterwards, both groups took a vision test. The group that drank orange juice had better vision than the water drinking group. Which of the following questions would provide information that would strengthen the conclusion that drinking orange juice improves vision? Were the groups created by randomly assigning people to their group? Did the participants know the purpose of the study? Both Neither

The answer is the first one: Were the groups created by randomly assigning people to their group?

Explanation for each option:

1. Were the groups created by randomly assigning people to their group?
   This question is crucial because random assignment helps ensure that the groups are comparable at the start of the experiment. It minimizes the effects of confounding variables and biases, making it more likely that any differences in outcomes (such as vision improvement) are due to the treatment (drinking orange juice) rather than pre-existing differences between the groups. Therefore, knowing that the groups were randomly assigned would strengthen the conclusion that drinking orange juice improves vision.

2. Did the participants know the purpose of the study?
   While this question is relevant to the study's design, it primarily addresses the potential for bias due to participants' expectations (placebo effect). Knowing whether participants were aware of the study's purpose could help assess the validity of the results, but it does not directly strengthen the conclusion about the effect of orange juice on vision. Therefore, this question is less critical than the first one for strengthening the conclusion.

3. Both
   While both questions are relevant to the study's design, the first question about random assignment is more directly related to strengthening the conclusion about causality. Therefore, "both" is not the best choice.

4. Neither
   This option is incorrect because the first question about random assignment is indeed relevant and important for strengthening the conclusion.

In summary, the most important question to strengthen the conclusion that drinking orange juice improves vision is whether the groups were created by randomly assigning people to their group.