Testing the interval-level measurement property of multi-item visual analogue scales.
BACKGROUND
Conditions were studied that may invalidate health-state values derived from the visual analogue scale (VAS).


METHODS
Respondents were asked to place cards with descriptions of EQ-5D health states on a 20 cm EuroQol VAS and modified versions of it, positioning them such that the distances between the states reflect their valuation for these states. Anchor-point bias was examined using the standard EuroQol VAS (n = 212) and a modified version (n = 97) with a different lower anchor. Context bias was examined in another group of respondents (n = 112) who valued three different sets of EQ-5D health states. Marker bias was studied in yet another group of respondents (n = 100) who placed the same EQ-5D states on the standard EuroQol VAS and on a modified VAS without anchors, categories, or measurement markers.


RESULTS
No indication for the existence of the anchor-point and the marker bias was found. However, the VAS valuations were significantly affected by the context of the set of health states in the scaling task.


CONCLUSIONS
Advanced methodologies should be incorporated in VAS valuation studies to deal with the context bias.
Krabbe
Paul F M
PF
Department of Medical Technology Assessment (138), Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands. p.krabbe@mta.umcn.nl
Stalmeier
Peep F M
PF
Lamers
Leida M
LM
Busschbach
Jan J V
JJ
D000293:Adolescent; D000328:Adult; D000368:Aged; D005260:Female; D006304:Health Status; D006305:Health Status Indicators; D006801:Humans; D008297:Male; D008875:Middle Aged; D010147:Pain Measurement / Q000592:standards*