1. A method of estimating a posterior probability for a switching state space model, the posterior probability providing the likelihood of a set of speech units and a set of hidden parameters for a sequence of frames based upon input values associated with the sequence of frames, the method comprising: approximating the posterior probability in part by multiplying individual hidden parameter probabilities together to form a product of the hidden parameter probabilities, wherein each individual hidden parameter probability provides the probability of a hidden parameter for a frame given a speech unit of the frame and given the input values for the sequence of frames; and adjusting parameters that define the hidden parameter probabilities so that the hidden parameter probabilities provide a better approximation of the posterior probability.