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= EQ-5D = EuroQol five dimensions questionnaire (EQ-5D) is a standardized instrument for measuring generic health status. The health status measured with EQ-5D is used for estimating preference weight for that health status, then by combining the weight with time, quality-adjusted life year (QALY) can be computed. QALYs gained is used as an outcome in cost-utility analysis which is a type of economic evaluation that compares the benefit and cost of health care programs or interventions. Many countries generated a value set (preference weights) of their own population and have used it for estimating QALY to make decisions in resource allocation. There are currently 171 language versions of EQ-5D questionnaire available. EQ-5D is one of the most commonly used generic health status measurement, and its good validity and reliability have been reported in various health conditions.

Development
EQ-5D was first introduced in 1990 by the EuroQol Group. This group was initially formed in 1987 with the researchers of multidisciplinary areas from five European countries; Netherlands, UK, Sweden, Finland, and Norway. They worked cooperatively with the aim of developing an instrument which is not specific to disease but standardized and can be used as a complement for existing health-related quality of life (HRQoL) measures. Other required characteristics of the new instrument were capable of being sent as a postal questionnaire for self-completion, easy to complete, applicable to everyone, can produce a single index value, and can take into account the health status ‘worse than dead’.

Components
The EQ-5D questionnaire is made up for two components; health state description and evaluation.

In description part, health status is measured in terms of five dimensions (5D); mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. Mobility dimension asks about the person's walking ability. Self-care dimension asks about the ability to wash or dress by oneself, and usual activities dimension measures performance in "work, study, housework, family or leisure activities". In pain/discomfort dimension, it asks how much pain or discomfort they have, and in anxiety/depression dimension, it asks how much anxious or depressed they are. The respondents self-rate their level of severity for each dimension using three-level (EQ-5D-3L) or five-level (EQ-5D-5L) scale.

In evaluation part, the respondents evaluate their overall health status using the visual analogue scale (EQ-VAS).

EQ-5D-3L
When EQ-5D was first developed, the scale used in the health state description part was three-level; having no problems, having some or moderate problems, being unable to do/having extreme problems. As an example, three levels of mobility dimension are phrased as "I have no problems in walking about", "I have some problems in walking about", and "I am confined to bed". The respondents are asked to choose one of the statements which best describes their health status of surveyed day. Rated level can be coded as a number 1, 2, or 3, which indicates having no problems for 1, having some problems for 2, and having extreme problems for 3. As a result, a person's health status can be defined by a 5-digit number, ranging from 11111 (having no problems in all dimensions) to 33333 (having extreme problems in all dimensions). 12321 indicates having no problems in mobility and anxiety/depression, having slight problems in self-care and pain/discomfort, and having extreme problems in usual activities. There are potentially 243 (=35) different health states.

EQ-5D-5L
Although its brevity contributed a lot for the wide use of EQ-5D, the three-level scale showed some limitations. The major drawback is that it has much fewer descriptive capability of health status compared to other generic instruments. For example, the Health Utility Index Mark 2 and Mark 3 (HUI 2 and HUI 3) and the Short Form 6D (SF-6D) can define 24,000, 972,000, and 18,000 unique health states, while EQ-5D-3L can do only 243. As a consequence, it suffers from ceiling and flowing effect and showed low sensitivity to catch small changes of health status especially when used to patients with milder conditions. To improve such constraints of the three-level scale, the new version of EQ-5D with five-level scale was developed (EQ-5D-5L). The number of levels of severity was increased to five in this new version; having no problems, having slight problems, having moderate problems, having severe problems and being unable to do/having extreme problems. The new version can define 3,125 (=55) different health states. Some of the wordings of the scale were revisited to be clearer and the instruction for EQ-VAS was simplified. No changes were made for the five dimensions.

The validity and reliability of the EQ-5D have been assessed for the different language versions and various health conditions, including cancer, type 2 diabetes, COPD, asthma, and cardiovascular disease, and so on. The 5L system showed improved responsiveness compared to the 3L system, and also good validity and reliability. EQ-5D-5L has also been recommended to the elderly population as a generic health status measurement, in combination with other supplementary measurements to capture all related aspects in their quality-of-life.

Visual analogue scale (EQ-VAS)
Visual analogue scale is the second part of the questionnaire, asking to mark health status on the day of the interview on a 20 cm vertical scale with end points of 0 and 100. There are notes at the both ends of the scale that the bottom rate (0) corresponds to " the worst health you can imagine", and the highest rate (100) corresponds to "the best health you can imagine". In the EQ-5D-3L version, the respondents has to draw a line from the box on the questionnaire to the scale indicates the health state of the interviewed day, while the EQ-5D-5L version asks to mark X on the scale to indicate the today's health and write the number of the scale marked in the empty box on the questionnaire. A well-known limitation of visual analogue scale is end-of-scale bias that respondents are less likely to use the extreme ends of the scale for rating their health status. However, it is still useful and the simplest direct method for valuing health-related quality of life (HRQoL) weights.

Valuation process
Once the health status is assessed from the description part, the 5-digit number can be converted into a preference weight which is also referred to as a single weighted index score. It can be done by using methods for generating HRQoL weights, such as visual analogue scale (VAS), the time-trade-off (TTO), and the standard gamble (SG). The choice of the valuation methods can vary. The initial purpose of having visual analogue scale in the EQ-5D questionnaire was to get preference weights using the scale, but the time-trade-off has become favored because it is a "choice task", not a "rating task" which easily involves some scaling bias. Time-trade-off is recommended when performing cost-utility analysis using quality-adjusted life year (QALY) as an outcome, but other methods could be chosen for different type of analyses. A value set was developed using time-trade-off in many countries, including the United Kingdom, United States, Spain, Japan, and Germany. The index score of a value set derived from the general population sample can be regarded as a "societal valuation of the respondent's health state" in that country. In contrast, the scores from the visual analogue scale in the questionnaire indicates the respondent's own assessment of his/her health status".