Look out for treatment comparisons where people’s outcomes were not counted in the group to which they were assigned.
Deciding who gets which treatment by chance (randomly) – something like pulling out names from a hat or flipping a coin – helps to ensure that people in comparison groups are similar before they receive treatment.
However, sometimes people in a study do not receive or take the treatment to which they were assigned. Those people may be different from those who do take the treatment to which they were assigned. So, not including them in the results may mean that the people in the comparison groups are no longer similar. To make sure that the comparison groups are similar in the results, and the comparison is fair, people should be counted in the group to which they were assigned.
For example, in a comparison of surgery and a medicine, people who die while waiting for surgery should be counted in the surgery group, even though they did not receive surgery. Not doing this would make surgery appear better than it actually is.
People not taking the treatment to which they were assigned may make the treatment appear less effective than it would if everyone had taken it. But there is less chance of the results being misleading because of dissimilar comparison groups.
REMEMBER: Think about whether people’s outcomes were counted in the treatment group to which they were assigned.