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. Conclusion: Advanced methodologies should be incorporated in VAS valuation studies to deal with the context bias.

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