What Determines the Shape of an EQ-5D Index Distribution?

Abstract

Background: Distributions of EQ-5D index values in patient and general populations typically have a non-normal distribution, divided into two distinct groups. It is important to understand to what extent this is determined by the way that the EQ-5D index is constructed rather than by the true distribution of ill-health.

Aims: This paper examines the determinants of the ‘two groups’ distribution typical of EQ-5D index data. We examine the extent to which the distribution is attributable to properties of the EQ-5D classification system used to create health state profiles or to the properties of the weights applied to profiles.

Methods: We analyse data from the English NHS PROMs programme (hip and knee replacements and varicose vein and hernia repairs) and from a study of two chronic conditions (asthma and angina). The distributions of EQ-5D index values are compared with distributions from which weights have been stripped, and profile data decomposed into their constituent dimensions and levels. They are also compared with condition-specific indexes and the distributions of EQ-5D indexes using different country weights, both Time Trade-Off and Visual Analogue Scale based.

Results: The EQ-5D picks out differences between patients in respect of dimensions that are mainly observed at level 2 or 3. The weights commonly used to calculate the index exacerbate this grouping by placing a larger weight on level 3 observations, and generate a noticeable gap in index values between the groups.

Conclusions: It is both important and informative to undertake exploratory data analysis on EQ-5D data. The analytical methods used for this may be simple. Concentrating on the EQ-5D index in effect obscures useful information about health states and may even produce misleading information.