['NI margins have to be chosen appropriately to control the risk of degradation of treatment effects in non-inferiority (NI) trials. We aimed to study whether the current choice of NI margins protects sufficiently against a degradation of treatment effect on an average.', 'NI trials reflecting current practice were assembled and for each trial, the NI margin was translated into a likelihood of degradation. The likelihood of degradation was calculated as the conditional probability of a treatment being harmful given that it is declared non-inferior in the trial, using simulation. Its distribution among the NI trials was then studied to assess the potential risk of degradation.', "The median (lower/upper quartile) NI margin among 112 binary outcome NI trials corresponded to an odds ratio of 0.57(0.45, 0.66), while among 38 NI trials with continuous outcome, to a Cohen's d of -0.42(-0.54, -0.31) and a hazard ratio of 0.82(0.73, 0.86) among 24 survival outcome NI trials. Overall, the median likelihood of degradation was 56% (45%, 62%)."] Does choice of non-inferiority ( NI ) margins protect against degradation of treatment effects on an average -- an observational study of registered and published NI trials?