The study looked at how well a method called SFA estimates inefficiency in postal services. They used simulated data to see how unobserved differences affect the estimates in both cross section and panel data. The results showed that inefficiency estimates can be quite different between the two types of data when there are hidden differences. This matches what was found in real postal sector data. The study also suggested a new way to estimate costs when inefficiency stays the same over time and there are hidden differences.