Size matters: EQ-5D in transition.

Progress in the world of science is seldom trouble-free, linear, nor sometimes even recognized for what it is. Where physical entities are the focus of our studies, there is at least scope for the type of observation and measurement that is denied those whose research is centered on less tangible phenomena. So it is with the investigation of health-related quality of life (HrQoL). Here we are dealing with a construct for which there is no universally agreed definition and no standard mechanism for its translation into practice. We lack an objective measure of health status against which to "calibrate" new instruments designed to measure it. As a consequence, attributes such as "precision" and "accuracy" can only be regarded as being relative in nature. When comparison is made between competing HrQoL measures, we should remember that we have only sketchy evidence about the "true" distribution of this construct when applied to the general population or to important subgroups. So, can techniques such as Rasch analysis help rescue us from this quagmire? Pickard et al1 clearly believe so. Others may wish to test its usefulness as a "crosswalk" device for establishing correspondence of value. The focus of this commentary is rather on the nature of the problem confronting us when we seek to describe health. So let us start at the beginning. EQ-5D is one of the most widely used generic index measures of health-related quality of life. Like its near neighbor and rival, the Health Utilities Index, EQ-5D is made up of 2 discrete but related components a descriptive system that defines a finite number of health states or profiles and a corresponding weighting system that enables values to be associated with each of those states. EQ-5D has been the subject of a sustained program of research conducted by members of the EuroQoL Group2 a research network now approaching its 20th anniversary. Something like 1000 man-years of research has been collectively invested in studying one or the other of the myriad elements that make up this apparently simple measurement technology. This program of research has investigated a diverse range of issues such as the effect of varying the size and orientation of graphical rating scales, the reliability of recalled assessment of health status, as well as pursuing the Group's central task of investigating the value for health and its variation across national boundaries. EQ-5D has remained unchanged in format since its designation by the EuroQoL Group in 1993 after 6 years of collective experimentation. It is based on a 5-dimensional classification of health defined in terms of mobility, self-care, usual activities, pain/ discomfort, and anxiety/depression. Each dimension has 3 possible response categories corresponding to no problem/some problem/extreme problem. Taken together these define a total of 243 health states. Standard valuation sets for these states are available for several countries,3 with the most recent of these being generated for the United States.4 One reservation that has sometimes been expressed in respect of the EQ-5D classification is that it is relatively crude in terms of its descriptive design and that movement from "no problem" to "slight problem" is accompanied by a relatively large reduction in value, in particular the value drop of around 0.15 from full health to the next mildest health state.