(a) Classification methodology. (1) A patient classification system is used to classify patients in inpatient rehabilitation facilities into mutually exclusive case-mix groups.

(2) For purposes of this subpart, case-mix groups are classes of Medicare patient discharges by functional-related groups that are based on a patient's impairment, age, comorbidities, functional capabilities, and other factors that may improve the ability of the functional-related groups to estimate variations in resource use.

(3) Data from admission assessments under § 412.610(c)(1) are used to classify a Medicare patient into an appropriate case-mix group.

(4) Data from the discharge assessment under § 412.610(c)(2) are used to determine the weighting factors under paragraph (b)(4) of this section.

(b) Weighting factors—(1) General. An appropriate weight is assigned to each case-mix group that measures the relative difference in facility resource intensity among the various case-mix groups.

(2) Short-stay outliers. We will determine a weighting factor or factors for patients that are discharged and not transferred (as defined in § 412.602) within a number of days from admission as specified by us.

(3) Patients who expire. We will determine a weighting factor or factors for patients who expire within a number of days from admission as specified by us.

(4) Comorbidities. We will determine a weighting factor or factors to account for the presence of a comorbidity, as defined in § 412.602, that is relevant to resource use in the classification system.

(c) Revision of case-mix group classifications and weighting factors. We may periodically adjust the case-mix groups and weighting factors to reflect changes in—

(1) Treatment patterns;

(2) Technology;

(3) Number of discharges; and

(4) Other factors affecting the relative use of resources.