Look out for treatment comparisons where what happened was measured differently in the comparison groups.
Sometimes, what happened (outcomes) are measured or detected differently in two treatment comparison groups. This makes it is hard to know how much differences in outcomes between the two groups were because of how they were measured and how much they were because of real differences in the effects of the treatments that were compared.
For example, if the people checking the outcomes believe that one of the treatments is better and they know who got that treatment, they may be more likely to see better outcomes in those people.
REMEMBER: Ask if outcomes were measured the same way in the groups that were compared.