The outcomes should be assessed in the same way for all animals in a study.
The only difference between the groups should be the intervention being tested. This includes how the outcomes are measured. If the outcomes are not measured in a standardised way 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 different measurements.
One way to address this issue is to “blind” the treatment groups so the researchers do not know which group each animal belongs to. This method helps to combat interpretation bias within the study.
REMEMBER: Fair comparisons can only be made when groups are assessed in the same way.