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