Hi all, I am currently working with a very large set of differential equations; it is a disease model with 20160 classes depending on which vaccine has been admnistered, history of infection, and age. To solve this I have been using the runge_kutta_fehlber78 controlled stepper with the integrate_const function. There I need to set the absolute_error, relative_error and the step_size. My issue is that the results from the solver seems to depend highly on the step_size and whether or not an observer is provided. Since the solver is adaptive I feel like the results should not depend on step_size and they should definitely not depend on the presence of an observer. My question is why would the results be related to the step_size or the observer? How would it be possible to get the same results regardless of the step_size and the presence of an observer? Could this be related to the system being stiff and if so how would it be possible to solve it (The jacobian is too complicated to calculate because of the population dynamics)? Any help would be appreciated. Many thanks, Jules