Given the high cost of medical care, research that points the way to avoid illness is welcome. Previously performed research tells us that stress affects the immune system. Two scientists at Carnegie Mellon Hospital in Pittsburgh asked 114 healthy adults about their social circles; they were asked to list every group they had contact with at least once every 2 weeks—family, coworkers, neighbors, friends, religious groups, and community groups. Participants also reported negative life events over the past year, events such as death of a friend or relative, divorce, or job-related problems. The participants were divided into four groups:

Group 1: Highly social and highly stressed

Group 2: Not highly social and highly stressed

Group 3: Highly social and not highly stressed

Group 4: Not highly social and not highly stressed

Each individual was classified in this way. In addition, whether each person contracted a cold over the next 12 weeks was recorded (1 = Cold, 2 = No cold). Can we infer that there are differences between the four groups in terms of contracting a cold?