Look out for comparisons of treatments between studies that are different.
For many conditions (e.g. depression) there are more than two treatments (for example, different medicines, and different types of psychotherapy). All the possible treatments for a condition 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 drug A with ‘usual care’ and comparisons of drug B with ‘usual care’, but no studies that compare drug A with drug B directly. In this case, people making a decision about what drug to use might indirectly compare drug A with drug B by examining how each drug compared to ‘usual care’.
However, there can be important differences between studies when treatments are indirectly compared. For example, the participants might have been more or less sick in the drug A studies or ‘usual care’ might have been more or less effective in the those studies. These differences between the studies can make the difference in outcomes for drug A and drug 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.