Why observational studies are important in comparative effectiveness research: the effect of breast-conserving therapy and mastectomy in the real world

We thank Dr. Franceschini and colleagues for their comment (1) on our recently published article in the Lancet Oncology (2). First, it is completely justified to ask oneself whether results from observational studies are likely to arise from selection bias. It is extensively described that observational studies investigating treatment effects are prone to confounding by severity (3). However, in contrast to randomised controlled trials (RCTs), observational studies do reflect the real-world population. As long as we interpret the results carefully, as it should be done for RCTs as well, these types of studies are of additional value through their ability to include large numbers of patients in conditions as we see them in daily practice (4,5).