Apart from the interventions being studied, all other factors, including animal groups, treatments, and study conditions for the groups being compared should be the same.
For comparison groups to be fair several factors must remain similar between the different groups in the study.
The animals in each group should be as similar as possible at the start of the study, they should be cared for similarly and outcomes should be measured similarly across all groups. The only difference between the groups should be the intervention being tested.
If these factors are not kept constant between the groups, then we cannot be sure if the results are due to the intervention or if the results are being affected by the differences.
A good way to ensure this is for participants to be allocated to each group randomly and for the study to be conducted in a blind format. If the researchers do not know which treatment a group is receiving, then they cannot treat them differently.
REMEMBER: It is important to consider any differences that may arise between different groups within the study and how this might influence any results obtained.