title: Crosswalk algorithms for the conversion of mean EQ-5D indices calculated with different value sets.
abstract: OBJECTIVE
  The EQ-5D questionnaire of EuroQol is an important tool, not only for researchers, but also for quality registers. Until 2013, Sweden did not have a country-specific value set to convert the EQ-5D health states into a single index. Commonly, the UK time trade-off (UK TTO) value set has been used. The study reported here aimed to establish an easy to use tool for the bidirectional crosswalk of the mean EQ-5D values based on two different value sets: the UK TTO and the Swedish TTO value sets.
  
  
  METHODS
  Using an artificial data set encompassing all possible EQ-5D outcomes, we calculated the EQ-5D index using both the UK TTO and Swedish TTO value sets. Thereafter we modelled the relationship between the two indices using least-squares regression and major axis regression. A series of simulations was run to assess the feasibility of the obtained crosswalk algorithms.
  
  
  RESULTS
  Major axis regression was superior to ordinary least-squares regression. Converting the mean EQ-5D values from the UK TTO scale to the Swedish TTO scale was more accurate than the inverse conversion. Values close to the floor and ceiling of the EQ-5D index were more challenging to crosswalk. CONCLUSIONS WE ESTABLISHED ALGORITHMS BASED ON MAJOR AXIS REGRESSION TO CROSSWALK EQ-5D VALUES BASED ON THE UK TTO AND THE SWEDISH TTO VALUE SETS THE PRESENTED ALGORITHM MAY FACILITATE COMPARISONS OF EQ-5D VALUES WHEN ONLY MEAN VALUES ARE AVAILABLE THE EXISTENCE OF A CROSSWALK ALGORITHM WILL EASE THE TRANSITION FROM THE UK TTO TO THE SWEDISH TTO VALUE SET.
authors.affiliation: Swedish Hip Arthroplasty Register, Sweden Department of Orthopaedics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Sweden szilard.nemes@registercentrum.se.
authors.forename: Szilárd
authors.initials: S
authors.lastname: Nemes
authors.affiliation: Swedish Hip Arthroplasty Register, Sweden Department of Orthopaedics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Sweden.
authors.forename: Göran
authors.initials: G
authors.lastname: Garellick
authors.affiliation: Swedish Hip Arthroplasty Register, Sweden.
authors.forename: Roger
authors.initials: R
authors.lastname: Salomonsson
authors.affiliation: Swedish Hip Arthroplasty Register, Sweden Department of Orthopaedics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Sweden.
authors.forename: Ola
authors.initials: O
authors.lastname: Rolfson
mesh_terms: D000465:Algorithms; D005240:Feasibility Studies; D006801:Humans; D015203:Reproducibility of Results; D011795:Surveys and Questionnaires; D013548:Sweden; D006113:United Kingdom
keywords: EQ-5D index; Swedish time-trade-off; UK time-trade-off; crosswalk; major axis regression