A method for determining an electrical property of interest of material(s) (205, 206) present in a target region in a process pipe or container, the target region being confined by or comprising a boundary surface formed by a body of the process pipe or container; the method comprising receiving measurement data representing measured values of a measurable electrical quantity dependent on the electrical property of interest of material(s) present in the target region, measured by a measurement probe having a plurality of measurement elements in a measurement connection with the target region; providing an observation model defining the relationship between observations of the measurable electrical quantity, corresponding to measurements made by the measurement probe, and the electrical property of interest of material(s) present in the target region; providing simulated observation data representing simulated values of the measurable electrical quantity produced by the observation model for an initial approximation of the electrical property of interest of material(s) present in the target region; determining an objective function comprising observation difference between the measured and the simulated values of the measurable electrical quantity as well as one or more prior models, and determining, on the basis of the objective function, an adjusted approximation of the electrical property of interest of materials present in the target region; and providing, on the basis of the adjusted approximation, estimate data representing an estimation of the electrical property of interest of material(s) present in a target region, characterized in that the method further comprises providing simulated statistics of a position deviation in the observations of the measurable electrical quantity, caused by a difference of an effective position of the measurement probe, defined relative to the boundary surface, from a predetermined reference position; and that the observation model is provided so as to define the observations of the measurable electrical quantity to correspond to measurements made with the measurement probe in the reference position, and depend on a position deviation which an effective position causes in the observations, the position deviation being determined to behave in accordance with the simulated statistics of a position deviation. receiving measurement data representing measured values of a measurable electrical quantity dependent on the electrical property of interest of material(s) present in the target region, measured by a measurement probe having a plurality of measurement elements in a measurement connection with the target region; providing an observation model defining the relationship between observations of the measurable electrical quantity, corresponding to measurements made by the measurement probe, and the electrical property of interest of material(s) present in the target region; providing simulated observation data representing simulated values of the measurable electrical quantity produced by the observation model for an initial approximation of the electrical property of interest of material(s) present in the target region; determining an objective function comprising observation difference between the measured and the simulated values of the measurable electrical quantity as well as one or more prior models, and determining, on the basis of the objective function, an adjusted approximation of the electrical property of interest of materials present in the target region; and providing, on the basis of the adjusted approximation, estimate data representing an estimation of the electrical property of interest of material(s) present in a target region, characterized in that the method further comprises providing simulated statistics of a position deviation in the observations of the measurable electrical quantity, caused by a difference of an effective position of the measurement probe, defined relative to the boundary surface, from a predetermined reference position;