In rare diseases, randomised controlled trials are not always feasible for ethical reasons or because the required number of patients is too large. In this case, the use of single arm trials (all patients in the treatment group) is preferred, and the control group may be taken from historical data (e.g., registries). Patients from registries are selected so that they respect the same inclusion/exclusion criteria present in the clinical trial. However, in the absence of randomisation, treatment and control groups may still have differences in baseline characteristics. It is necessary to take into account these differences in order to avoid, or limit, a bias in the treatment effect estimation.