than 200 languages. translation of the eq-5d into new languages is overseen by the euroqol group's version management committee (vmc) and is usually carried out in a collaboration with professional agencies with expertise in the cultural adaptation of pro measures. all translations follow a standard and closely monitored process that conforms to internationally recognised guidelines. the main steps in the translation process are forward translation, back translation, and cognitive debriefing (animation video: translating patient reported outcome measures for use around the world - the example of eq-5d). the goal of translation is to produce an eq-5d version that has the same meaning as the english source version. furthermore, it must sound natural and be easily understood in the target language. value sets for eq-5d health states when a person completes the eq-5d questionnaire, the descriptive system produces a 5-digit health status profile that represents that person's level of reported problems on the five eq-5d health dimensions. these profiles are usually referred to as ‘eq-5d health states’. as noted above, eq-5d-3l describes 243 potential health states while the eq-5d-5l describes 3125 potential health states. a numerical value can be attached to each eq-5d health state to reflect how good or bad a health state is according to the preferences of the general population of a country/region. health state values can also be referred to as eq-5d ‘index values’ or ‘index scores’. the collection of index values for all possible eq-5d states is called a value set. eq-5d value sets are constructed at the national level, reflecting the belief that preferences for health can differ across populations. valuation research aims to measure people's preferences with respect to health – in other words, how health is valued. this involves the participation of a representative sample of people from the general population in a standardised valuation experiment. in this experiment, participants are asked to value health by reviewing eq-5d health states. because eq-5d has values attached to its health states, it is widely used in the economic evaluation of health care interventions, where the convention is to measure health gains as value-weighted time using quality-adjusted life years (qalys). the values can also inform other research, such as studies in the burden of illness. various techniques can be used to assign a value to an eq-5d state. the euroqol group has done extensive research on different methods of valuing eq-5d health states (see the historical review below). most eq-5d value sets are based on the time trade-off (tto) approach, used either alone or in combination with discrete choice experiments (dce). eq-5d-3l value sets using tto are available for over 30 countries or regions. eq-5d-5l value sets using tto (+ dce) are available for many countries, and valuation studies are ongoing in further countries. eq-5d-3l value sets: vas and tto approaches valuation research in the late 1980s investigated several different methods, including ranking, magnitude estimation, paired comparisons as well as several forms of rating, as ways of obtaining values for health states and resulted in the choice of a vas approach. at that time, other methods such as tto and standard gamble (sg) were primarily associated with the development in canada of the health utilities index (hui). investigation into valuation methods continued, including a head:head comparison between tto and sg which resulted in the selection of tto for use in the 1993 measurement and valuation of health (mvh) study, a programme funded by the uk department of health and carried out by the university of york, uk. this generated an eq-5d value set based on tto values from the general public that could be used to generate qalys. the mvh value set became widely applied in economic evaluations in the uk and other countries, and continues to be used today. over the following years, eq-5d-3l value sets were produced for many countries using either tto and/or vas approaches . over time, however, tto emerged as the method of first choice. the eq-net project of 1998–2001, which was funded by the biomed programme of the european commission, resulted in the establishment of tto and vas databases to facilitate international comparisons of health state values. in 2009, the mvh protocol for valuation of eq-5d-3l health states was refined to improve the data collection process; this was referred to as the ‘paris protocol’. this protocol has been used in a number of eq-5d-3l valuation studies, including those of south korea, china, france, portugal, and brazil. the development of the eq-5d-5l renewed interest in the methodology of valuing health. until 5l value sets were produced, interim values for the eq-5d-5l were made available using a ‘crosswalk’ approach based on a study in six countries, in which the eq-5d-3l and 5l had been completed in parallel. this meant that values for eq-5d-5l health states could be derived from existing eq-5d-3l value sets. eq-5d-5l value sets: eq-vt combining composite tto and dce value sets for qaly calculations are required to range from 1 (representing full health) to 0 (representing dead), and it is generally accepted that very poor health states may be considered worse than being dead. due to the recognised challenges of valuing states worse than death, the euroqol group initiated a programme of research to further investigate valuation of these states. this involved testing new methods for tto, including multiple variants of ‘lead-time’ tto and ‘lag-time’ tto. it should also be noted that in the 5l valuation procedure euroqol continued to use tto, but vas was discarded and replaced by dce. with the introduction of 5l, the opportunity was taken to introduce a standardised protocol for the first time. this decision was based on multiple considerations. variability in how value sets were produced had provide much information about what worked and what did not work in the valuation of health, but it limited the comparability of value sets. moreover, valuation of 5l was considered more challenging than valuation of 3l – hence efforts to define best practice had to be renewed, to ensure high-quality data. furthermore, the programme of work that followed enabled several open methodological questions to be addressed. a key aspect in this research was the potential of computer-based technology to help guide the respondents through the valuation tasks. the programme led to a new tto approach (the composite tto; ctto) that removed the need for arbitrary rescaling of values for states worse than dead. composite tto uses the conventional approach for states considered better than dead, but lead-time tto for states considered worse than dead. at the same time, new approaches to health state valuation were being tested, such as dces. this work culminated in the eq-vt (euroqol valuation technology) protocol, which uses a standardised computer-assisted personal interview (capi) to derive health preferences for eq-5d-5l using ctto and dce . the first valuation studies using this approach were conducted in canada, china, england, the netherlands, and spain in 2012. following several refinements, the current eq-vt is labelled version 2.1. by september 2019, the eq-vt had been used in 34 countries around the world. eq-5d-y value sets due to the added difficulties of obtaining values for health states in paediatric populations, value sets for the eq-5d-y have taken longer to develop. in 2020, the euroqol group published an international valuation protocol for the youth version and the first eq-5d-y valuation study, for slovenia, was published in 2021. further eq-5d-y valuation studies are underway in countries around the world. note that eq-5d-3l value sets should not be used to assign values to eq-5d-y health states. eq-5d data analysis more detailed information on how to analyse eq-5d data is available in a book that describes various analytical approaches for the eq-5d descriptive profiles, eq vas, and eq-5d value sets. eq-5d data can be analysed in several different formats - per dimension, as a descriptive health state, as a weighted index on a 0-1 scale and as an overall measure of self-assessed health status (eq vas). it is important to consider these different formats separately, especially when reaching a judgement regarding the psychometric properties of the instrument as a whole. theoretical and psychometric properties the validity, reliability, and responsiveness of eq-5d-3l have been assessed for eq-5d versions in many different health conditions, including cardiovascular disease, mental health populations, aged care, skin conditions, cancer, and total knee arthroplasty. the brevity of eq-5d-3l is likely to be a major factor in its widespread use, but the three-level format can be a limitation in some clinical areas. the eq-5d-3l descriptive system can be less sensitive to small and medium changes in health status, so it may be less able to detect change in some conditions. the 5l descriptive system has shown improved responsiveness compared to the 3l system, e.g. in a comparison of eight patient groups, osteoarthritis, and joint replacement, as well as acceptable psychometric properties in conditions such as stroke, chronic obstructive lung disease, and scoliosis. some studies have assessed the eq-5d-3l and eq-5d-5l together. the psychometric properties of eq-5d-y have been assessed in a variety of settings and conditions. a value set derived from a general population sample has been criticized when used for economic evaluation for a lack of a compelling theoretical support however, such "experienced" value sets are nonetheless relevant in clinical settings or in population health studies (see below) where social preference weighted index scores are contraindicated. eq-5d in population health studies eq-5d has been extensively utilised in population health studies, both at the national level and for subsections of a country's population, such as a specific region. data collected with eq-5d can be used to assess and compare health status between groups of patients, between patients and the general population, or between the general populations of different countries. it can also be used to monitor changes in health status over time at population level. self-reported health as captured by eq vas comes directly from the respondent and perfectly meets the published fda criteria for patient reported outcome (pro) measures. imposing social preference weights on such data violates the pro condition and creates obstacles in cross-national comparisons of population health. cross-country analyses of self-reported population health using eq-5d data have been reported and subsequently updated. the first publication incorporated data from 15 countries, and the second from 24 countries. since then, eq-5d-5l population norms have also been published for japanese, german, and south australian populations. eq-5d in economic evaluation eq-5d is the most widely used health-related quality of life questionnaire in health economic evaluations. eq-5d can be used to derive a set of values that reflect people's opinions of the relative importance of different health problems. these values can be used to derive qalys for application in cost-effectiveness and cost-utility evaluations. in a review study of recommendations from national hta agencies, eq-5d, hui, and sf-6d were the three multi-attribute utility instruments (mauis) most frequently mentioned in hta guidelines. the most commonly-cited maui (in 85% of pharmacoeconomic guidelines) was the index form of eq-5d, either as a preferred maui or as an example of a suitable maui for use in cost-utility analysis in hta.