Evidence-appraisal glossary
Transitivity
Transitivity is the assumption behind indirect comparison: that studies comparing A to B and B to C are similar enough in their patients and settings that B can serve as a shared bridge to compare A and C. If the trials differ in ways that affect the outcome, the bridge is unreliable.
Also called: transitivity assumption, exchangeability of comparisons.
A network meta-analysis compares treatments that were never tested head to head by chaining together the trials that share a common comparator. That chaining only holds if the trials are interchangeable in every factor that could modify the effect, such as disease severity, dose, or length of follow-up. When transitivity is broken, the indirect estimate can be biased in ways that no amount of statistical precision will reveal, so appraising it means examining the clinical and methodological features of the contributing trials, not just the numbers.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.