Unfair comparison

Dissimilar comparison groups

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

For a comparison of interventions to be fair, 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, in an intervention comparison, if the group that gets a intervention is in better condition at the beginning of the study than the group that does not get that intervention, that could make the intervention look like it works better than it actually does.

The best way to make sure that both groups are alike is to decide which animals/plants/sites get which intervention by chance (randomly) – something like pulling out names from a hat or flipping a coin.

REMEMBER: Think about whether the decision of which intervention or comparison was applied to each individual/site was by chance (randomly) in a intervention comparison, and if the indivuals/sites in the comparison groups were similar at the start of the study.