A study reports a relative risk increase of 2 of developing heart arrhythmia for those drinking two cups of coffee per day.
The study was performed in 100,000 people, randomised into groups A and B with 50,000 each. Group A had 2 cups of coffee per day and group B didn’t have any coffee at all. In group A, two people developed heart arrhythmia, whereas in group B, one person did.
Which feature of these results is the most problematic when trying to draw conclusions?