One embodiment relates to a medical device including a sensor 40 to provide signals indicative of a detected posture state of a patient. A memory 36 stores one or more defined vectors, each defined vector being associated with a tolerance describing a relationship with the defined vector. A processor 34 determines a respective similarity between the detected vector and each of one or more of the defined vectors and classifies a posture state of the patient based on whether any similarity has a relationship to the respective defined vector that is described by the associated tolerance. In one embodiment, the similarity is determined without regard to a coordinate system of the patient. Another embodiment relates to determining the similarity based on at least one of an inner product, a length of the defined posture vector and a length of the detected vector.
