Dear editor, First, we would like to thank the editorial board for the interest in our study and the thorough and extensive commentary [18] to our publication, “The impact of provider surgical volumes on survival in children with primary tumours of the central nervous system—a population-based study” [48]. The editorial includes comments on central publications and addresses some of the problems with the available evidence concerning the volume-outcome relationship. We appreciate the opportunity to comment further on the argumentation for and against centralization of paediatric neurosurgery. If a surgeon does not practice surgery, he or she will not master the procedures. This is self-evident. This is why training and licensing is required to become specialists and subspecialists. Although, accepting a volume-outcome relationship, there are still many unanswered questions as to what to gain from centralisation of different neurosurgical procedures. Here, we would like to expand the literary review in the editorial and comment on the specific points in respect to our study. The magnitude and importance of the volume-outcome relationship is difficult to assess from a review of the available literature. Major limitations rise from the fact that the data source are mainly American administrative claims databases or discharge summaries of variable quality that do not contain much clinical data. Usually, such record data only allow for crude adjustments for age, sex, ethnicity, hospital size and socioeconomic status [often just assessed as mean income in the postal code of the patient’s residential address (!)]. The severity and incidence of the treated disease is not corrected for, and there are often considerable limitations in data concerning differences in referral and case mix [28, 29]. It should further be remembered that high volume institutions in the USA more often treat younger patients, whites, patients with private insurance and residents of wealthier areas. Hospitals with the highest volumes typically also have more elective cases. In publications based on American administrative databases, the main outcome is usually surgical mortality, measured as in-house-mortality. Although 30-day mortality has a better face validity, the correlation to in-house mortality rates are generally at least moderate (kappa > 0.40), but with variance across conditions [14]. Surgical mortality has in the USA been endorsed as an Inpatient Quality Indicator (IQI) by the Agency for Healthcare Research and Quality in eight surgical procedures for adults, including craniotomies (the other seven are specific surgical procedures, while craniotomy is the opening of a body part). The line of logic is that since (1) surgical mortality in craniotomies is defined as a quality indicator, and (2) surgical mortality in many publications is lower in high-volume providers of care, then (3) surgical volume is a quality indicator (quod erat demonstrandum). However, studies by the Veterans Affair (VA) National Surgical Quality Improvement Program (NSQIP), which prospectively collects clinical data on all major surgical operations in the VA, do not reach the same conclusions as studies based on administrative claims data: “Unlike retrospective studies that are based on administrative databases, NSQIP studies have failed to demonstrate a direct relationship between volume and risk-adjusted outcomes of surgery across various specialties. These studies have emphasized that the quality of systems of care was more important than volume in determining the overall quality of surgical care at an institution. High-volume hospitals could still deliver poor care in as much as low-volume hospitals could deliver good care. NSQIP studies have also underscored the major limitations of claims data and administrative databases in the provision of adequate risk-adjustment models that are crucial for volume-outcome studies” [35].