Unfair comparison

Dissimilar comparison groups

Look out for treatment comparisons where the comparison groups were not alike.

For a comparison of treatments to be fair, the people in the comparison groups should have been as similar as possible at the start of a study. If groups were not similar, this may have affected the results. We cannot know if what happened in the study was due to the groups being dissimilar, the interventions that were compared, or both.

For example, if the group that gets an intervention is better off (less depressed, less severe substance use) at the beginning of the study than the group who does not get that intervention, that could make the treatment look like it works better than it actually does.

The best way to make sure that the people in both groups are alike is to decide who gets which treatment by chance (randomly) – something like pulling out names from a hat or flipping a coin.

REMEMBER: Think about whether who got which intervention was decided by chance (randomly) in a treatment comparison, and if the people in the comparison groups were similar at the start of the study.