Look out for comparisons of interventions between studies that are different.
For many environmental problems there are more than two treatments (for example, different practical or incentive-based interventions). All the possible interventions for a problem are very rarely compared in a single study, so it may be necessary to consider indirect comparisons among treatments.
For example, there may be comparisons of intervention A with no intervention and comparisons of intervention B with no intervention, but no studies that compare intervention A with B directly. In this case, people making a decision about what intervention to use might indirectly compare A with B by examining how each drug compared to no intervention.
However, there can be important differences between studies when interventions are indirectly compared. For example, the subjects might have been more or less polluted in the Intervention A studies or no intervention might have been different in those studies. These differences between the studies can make the difference in outcomes for A and B seem smaller or larger than it actually is.
REMEMBER: When indirect comparisons are needed to inform treatment choices, think about whether careful consideration was given to differences between studies.