Comment: 14 Pages, to be published in JATMWe systematically study cornerstones that must be solved to define an air traffic control benchmarking system based on a Data Envelopment Analysis. Primarily, we examine the appropriate decision-making units, what to consider and what to avoid when choosing inputs and outputs in the case that several countries are included, and how we can identify and deal with outliers, like the Maastricht Service Provider. We argue that Air Navigation Service Providers would be a good choice of decision units within the European context. Based on that, we discuss candidates for DEA inputs and outputs and emphasize that monetary values should be excluded. We, further suggest to use super-efficiency DEA for eliminating outliers. In this context, we compare different DEA approaches and find that standard DEA is performing well.