An apparatus for determining the clinical state of a subject under anesthesia, the apparatus comprising: measurement means for acquiring measurement signals from the subject; transform means for subjecting the measurement signals to at least one adaptive transform, respectively, thereby to obtain at least two transformed measurement signals, wherein each adaptive transform is dependent on previously acquired history data; means for forming a diagnostic index which is dependent on the transformed measurement signals, the diagnostic index serving as a measure of the clinical state of the subject and providing an index of nociception (IoN); first monitoring means for detecting a predetermined event; and control means for replacing at least part of the previously acquired history data on which at least one of the at least one adaptive transform is currently dependent with other previously acquired history data, the control means being responsive to the first monitoring means; said apparatus further comprising second monitoring means for choosing at least one predetermined subject state category into which the subject is to be classified based on the subject's current state, said at least one subject state category being chosen from among at least two predetermined subject state categories, the first monitoring means being responsive to the second monitoring means for detecting the predetermined event when the at least one predetermined subject state category changes; wherein the other previously acquired history data is derived from data acquired for the at least two predetermined subject state categories; wherein the measurement means are configured to acquire at least two measurement signals from the subject, each of the at least two measurement signals being indicative of a respective physiological parameter; the transform means being configured to subject the at least two measurement signals to the at least one adaptive transform, respectively, thereby to obtain the at least two transformed measurement signals; and the means for forming the diagnostic index being configured to produce a weighted average of the at least two transformed measurement signals, and wherein the diagnostic index provides a quantitative measure of a current clinical state of the patient by applying a normalization transform to at least one of the measurement signals so that the index of nociception (IoN) provides a diagnostic scale on which a certain reading corresponds to the same level for all patients. measurement means for acquiring measurement signals from the subject; transform means for subjecting the measurement signals to at least one adaptive transform, respectively, thereby to obtain at least two transformed measurement signals, wherein each adaptive transform is dependent on previously acquired history data; means for forming a diagnostic index which is dependent on the transformed measurement signals, the diagnostic index serving as a measure of the clinical state of the subject and providing an index of nociception (IoN); first monitoring means for detecting a predetermined event; and control means for replacing at least part of the previously acquired history data on which at least one of the at least one adaptive transform is currently dependent with other previously acquired history data, the control means being responsive to the first monitoring means; said apparatus further comprising second monitoring means for choosing at least one predetermined subject state category into which the subject is to be classified based on the subject's current state, said at least one subject state category being chosen from among at least two predetermined subject state categories, the first monitoring means being responsive to the second monitoring means for detecting the predetermined event when the at least one predetermined subject state category changes; wherein the other previously acquired history data is derived from data acquired for the at least two predetermined subject state categories; wherein the measurement means are configured to acquire at least two measurement signals from the subject, each of the at least two measurement signals being indicative of a respective physiological parameter; the transform means being configured to subject the at least two measurement signals to the at least one adaptive transform, respectively, thereby to obtain the at least two transformed measurement signals; and the means for forming the diagnostic index being configured to produce a weighted average of the at least two transformed measurement signals, and wherein the diagnostic index provides a quantitative measure of a current clinical state of the patient by applying a normalization transform to at least one of the measurement signals so that the index of nociception (IoN) provides a diagnostic scale on which a certain reading corresponds to the same level for all patients.