Participants were instructed to take the medication as prescribed at the onset of a cold for the next 6 months. Half of the participants were randomly assigned to receive Vitamin C pills, and the other half were randomly assigned to receive placebo pills. Participants were instructed to report the date of the onset of a cold, and the date at which the cold had improved enough for the participant to participate in daily life activities (such as exercise, work, gatherings, etc.).

When reviewing the results, the researchers found that participants who exercised for more than 180 minutes per week did see results when taking Vitamin C. In the next study, the researchers plan to first divide the participants into two groups, those who exercise at least 180 minutes per week, and those who do not exercise at least 180 minutes per week. From these groups based on weekly exercise amount, the researchers will randomly assign participants within each group to be in either the Vitamin C group or the placebo group. Whether a participant exercises for at least 180 minutes per week or not will act as aln) [select from below} variable in the next study. 0 Confounding O Blocking 0 Response O Explanatory