Selective data entry
Selective Data Entry may be one of the flaws of the collected data.
As one of the goals of the database is the comparison of centers and surgeons, every center wants to gain the best results in the reports. This can be partially achieved by entering the data selectively, i.e. entering only the successful cases.
Though there is no way to prevent this (not recommended!) practice, hospitals have to at least admit that they didn't enter all of the cases. Hence the need for entering the declared number of operations in each year.