Method of open-loop and/or closed-loop control of a production plant set up to produce thermoplastic polymer, performed by one or more apparatuses, comprising: - detecting a pressure value in the production plant; - determining a correction factor; - determining a volume flow rate; - determining an estimated value indicative of a viscosity, based at least partly on the pressure value detected and based at least partly on a temperature dependence of a viscosity behavior of the thermoplastic polymer and the volume flow rate; and - determining a viscosity value indicative of the viscosity behavior of the thermoplastic polymer at least partly depending on the correction factor, where the correction factor is indicative of a plant state, and where the plant state is determined at least partly by at least one existing production parameter, where the correction factor corresponds to the result of a statistical calculation, where the statistical calculation is based on historical values for the relationship between the estimated value indicative of a viscosity and a real viscosity, and where a first pressure is detected at a first position, - a second pressure is detected at a second position, and - the pressure value corresponds to the difference between the first pressure and the second pressure, and wherein the method further comprises: - comparing the viscosity value determined and the viscosity value indicative of the viscosity behavior of the thermoplastic polymer with a target viscosity value; - determining at least one piece of control information based at least partly on the result of the comparison; - issuing or triggering the issuing of the particular piece of control information, with enabling of open-loop and/or closed-loop control of the production plant based on the particular piece of control information; and - controlling the production plant by open-loop and/or closed-loop control at least partly depending on the piece of control information. - detecting a pressure value in the production plant; - determining a correction factor; - determining a volume flow rate; - determining an estimated value indicative of a viscosity, based at least partly on the pressure value detected and based at least partly on a temperature dependence of a viscosity behavior of the thermoplastic polymer and the volume flow rate; and - determining a viscosity value indicative of the viscosity behavior of the thermoplastic polymer at least partly depending on the correction factor, where the correction factor is indicative of a plant state, and where the plant state is determined at least partly by at least one existing production parameter, where the correction factor corresponds to the result of a statistical calculation, where the statistical calculation is based on historical values for the relationship between the estimated value indicative of a viscosity and a real viscosity, and where a first pressure is detected at a first position, - a second pressure is detected at a second position, and - the pressure value corresponds to the difference between the first pressure and the second pressure, and wherein the method further comprises: - comparing the viscosity value determined and the viscosity value indicative of the viscosity behavior of the thermoplastic polymer with a target viscosity value; - determining at least one piece of control information based at least partly on the result of the comparison; - issuing or triggering the issuing of the particular piece of control information, with enabling of open-loop and/or closed-loop control of the production plant based on the particular piece of control information; and - controlling the production plant by open-loop and/or closed-loop control at least partly depending on the piece of control information.