Look out for treatment comparisons where what happened was not measured in lots of people.
Sometimes, what happened (outcomes) are not measured in some of the people in treatment comparisons, so those people are left out of the results. When lots of people were left out of the results, or there was a big difference between the comparison groups in how many people were left out, the results can make the effects of the treatments seem bigger or smaller than they are.
For example, people may stop taking part in a study comparing treatments because the treatment they got was not helping them, or because it was making them worse. If those people were left out of the results, the results could make the effects of that treatment seem bigger than they are.
REMEMBER: Ask if lots of people who started out in a study comparing treatments were missing in the results.