Look out for comparisons of interventions 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 interventions 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 psychotherapy A with ‘usual care’ and comparisons of psychotherapy B with ‘usual care’, but no studies that compare psychotherapy A with psychotherapy B directly. In this case, people deciding about which intervention to use might indirectly compare psychotherapy A with psychotherapy B by examining how each 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 depressed in the psychotherapy 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 different interventions, making the effects of the interventions seem smaller or larger than they actually are.
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