1. A method comprising: determining an analytical solution expressing a temporal change in probability that a component of a system is in a particular state of four possible states at a particular time after an initial time based on four explicitly recited initial probabilities corresponding to the four states and twelve explicitly recited rate constants for switching from one state of the four states to a different state of the four states, wherein any rate constant or initial probability may take on a value of zero; determining a non zero value associated with an external factor applied to the component for each of one or more of the twelve rate constants; determining a non zero value for each of one or more of the four initial probabilities; and determining a temporal change in a probability that the component is in a certain state of the four states based on the values for the twelve rate constants and on the values for the four initial probabilities and on an elapsed time and on the analytical solution.