Look out for comparisons where what happened was measured differently in the comparison groups.
Sometimes, outcomes were measured or detected differently in two treatment comparison groups in a study. When this is the case, it is hard to know how much the study results were due to the intervention or due to the differences in how outcomes were measured.
For example, the people measuring the outcomes in a comparison of treatments for depression might believe that one of the treatments is better. If they know who got that treatment, they may be more likely to think that those people were less depressed and rate their symptoms differently.
One way of keeping this from happening is not to let the people checking outcomes know which people got which intervention (to “blind” them).
REMEMBER: Think about whether outcomes were measured the same way in the groups that were compared.