A method for operating an exhaust gas system is provided. The method includes determining an amount of a reducing agent to be supplied to the exhaust gas of an engine and evaluating measurements which indicate a content of nitrogen oxides in the exhaust gas downstream of a catalytic device adapted to diminish the content of nitrogen oxides. A magnitude and a frequency of the measurements are taken into account in determining the amount of the reducing agent to be supplied. A plurality of measurements is captured during a predetermined period of time. A magnitude of a measurement captured within this period of time is related to a quantity derived from the respective magnitudes of the plurality of measurements. The related measurement is utilized to determine the amount of the reducing agent to be supplied. A control assembly for operating an exhaust gas system is also provided.