we run a Kalman filter or a common smoother how well does it work so a natural comparison would be something along the lines of let's say I have some dynamical system and I don't get to observe the state directly but I get to see some observations so I would run a Kalman filter but since I'm running an experiment that could say well okay let me actually give myself access to this state see what it is and see how precise my filter is how well does it track this state and that might be but for debugging and just understanding how