Objectives: Several instruments for measuring health-relatedquality of life (HRQOL) have been developed, and others are underconstruction. The problem is whether the different HRQOL measuresshow comparable results. We first compared the functionalrelationship of the Nottingham Health Profile (NHP) and the15-dimensional measure of HRQOL (15D) in hip and knee replacementpatients. The hypothesis was that condition or intervention does notaffect the functional relationship between NHP and 15D changes.

Methods: We assessed the agreement of the instruments bycomparing observed changes in the 15D and its fitted values derivedby regressing the 15D by the NHP dimensions. Patients (n = 452) wererecruited consecutively from seven Finnish orthopedic departmentsduring April 1991–May 1992. HRQOL was measured prior to surgery and6, 12, and 24 months postoperatively.

Results: There was a different functional relationship betweenthe HRQOL instruments in hip and knee patients; they agreed upon thedirection of changes in HRQOL in 84% and showed opposite signs in12%. The NHP showed significantly more improvement in quality oflife than the 15D.

Conclusions: The two instruments were in slight but significantdisagreement. Estimates of effectiveness can vary according to theinstrument used. Comparisons of effectiveness in healthcare programsmeasured by differing instruments need information on the functionaldiscrepancies between the instruments in the conditions andinterventions in which they are applied.