Look out for intervention comparisons where what happened was not measured in all of the original subjects.
Sometimes, outcomes were not measured in some of the subjects (animals, crops, fields etc.) in an intervention comparison, so those subjects were left out of the results of the study. This might be because they were excluded because of side effects, environmental factors, or because the intervention was not working. When lots of subjects are left out of the results, or there is a big difference between the comparison groups in how many were left out, the results can make the effects of the interventions seem bigger or smaller than they are.
REMEMBER: Think about whether lots of the subjects that were included at the beginning of the study were missing in the results stage.