Key concepts in clinical epidemiology: addressing and reporting sources of bias in randomized controlled trials.

Randomized controlled trials are widely considered the most robust design for evaluating the effects of clinical interventions. While generalisability is often limited, randomization aims to ensure that effects observed are genuine. However, there are common sources of bias, even in well-conducted trials, that pose a threat to this interpretation. The revised Cochrane risk-of-bias tool for trials (RoB 2) distinguishes five domains of bias that can affect the results of trials stemming from (1) the randomization process, (2) deviations from intended interventions, (3) missing outcome data, (4) outcome measurement, and (5) reporting of findings. We use RoB 2 as a framework for recommendations to help researchers mitigate these sources of bias and ensure transparency in reporting so that users of research are aware of them.