This study makes available the first Norwegian population norms for the EQ-5D-5L from the Norwegian general population. This data is highly important to Norwegian users of PROMs in clinical and health services research, and the Norwegian NQRs, where EQ-5D-5L is by far the most widely used patient-reported instrument. The survey was specifically designed for the collection of norm data, whereas published data for several countries followed the collection EQ-5D valuation data for national scoring algorithms [2, 14, 15, 17, 20] or as part of other health surveys [24, 25]. The former followed EuroQol Valuation Technology and survey requirements including computer-assisted face-to-face interviews and sample sizes of approximately 1000, although the EQ-5D-5L and EQ VAS are generally completed by means of pen and paper in this context [5, 14]. The presence of an interviewer may still contribute to social desirability bias .
Differences in survey design, methods of recruitment and reporting limit comparisons with EQ-5D-5L norms for those available from other countries. However, as was found for Norway, across the eleven countries for which EQ-5D dimension data was reported, older groups generally reported increasing problems apart from anxiety and depression [2, 14, 15, 17, 19, 20, 22-26]. There were some exceptions for the usual activities dimension in the age groups 50-79 years, which reported slightly less problems than the 40-49 age group. The findings are similar for age groups that overlap with those for 50-79 years in three Asian countries and Ireland [17, 19, 20, 26]. Furthermore, compared to the youngest age group, those aged 30-39 years reported less problems with usual activities, which is comparable to the findings from six countries [14, 15, 17, 20, 25, 26]. Higher levels of anxiety/depression in the youngest age groups were also found for the youngest age groups in five Asian countries and Slovenia [15, 17, 19, 21, 23, 26].
Apart from self-care, males generally reported less health problems than females across the EQ-5D-5L dimensions. Males in the age group 60-69 years reported slightly more problems than females for mobility. For three of the seven age categories (40-49, 60-69, 70-79), males reported more problems with self-care than females. Across the eight countries for which such data were reported, the findings were similar for two or three overlapping age categories for Bulgaria , China , Ireland  and Poland . For Indonesia, the same proportion of females and males reported no problems for self-care .
The use of national value sets for scoring the EQ-5D-5L limits the interpretation of EQ-5D index scores across countries. The additional use of a common value set, including the first EQ-5D-3L value set  with mapping , or summated rating scale based on the five items [20, 24], would aid interpretation but are rarely reported. EQ-5D index and EQ VAS scores did not consistently decrease with age, rather there was a slight increase for the second age group and two age groups from 50-69 years. Some increases in scores or leveling off with increases in age were found for ten countries reporting this data [15-21, 23, 25, 26]. In common with all other countries apart from the USA , EQ-5D index scores were lower for females than males [14-26]. The lower EQ VAS scores for males was previously found for four other countries [15, 20, 21, 25].
The use of postal administration followed published Norwegian surveys for collecting norm data for generic PROMs including the EQ-5D and SF-36, the most recent being reported in 2018 [11, 12, 29]. Independent of mode of administration, such surveys often have low response rates for older age groups, but the sampling methodology used here secured a relatively high number of respondents, which allowed the use of ten-year categories up to 80 years of age and over. Existing Norwegian surveys had smaller samples for older age groups [11, 12, 29], even when there has been a much larger sample [11,12]. This makes the norm data more relevant for the interpretation of EQ-5D scores from Norwegian patients, who are often older than respondents to surveys designed to give general population norms [27, 28]. Except for Spain, where the data came from a much larger sample , existing national norm data for the EQ-5D-5L has not included an age category of 80 years or above.
The response rate of 26% was low, and a reminder might have increased response rates. However, reminders sent to over 9,000 non-respondents to the first mailing would have made it costly. Based on published Norwegian surveys [11, 29], a low response rate was expected. The lottery incentive was included to mitigate this, albeit with what appears to be limited success. It is not possible to ascertain the impact of the lottery, but the most recent Norwegian postal survey designed to collect population norms for the SF-36, had a response rate of 20% before and 36% after one reminder . One postal survey designed to collect Norwegian norms for the earlier version of the EQ-5D, which has three levels (EQ-5D-3L), used a sample frame based on the same Norwegian register in 2010. No reminders were used, but a lottery ticket for NOK 25 (2.5 Euros) was given to half the sample, and the overall response rate was 23% .
The present study adopted a similar approach to other countries based on quota sampling including age and sex [20, 21]. In common with the vast majority of other countries [2, 14, 15, 18-23, 25-26], the data were not weighted to better approximate the characteristics of general population, and because the norms are shown by age and sex, there is no need for the sample to have the same distribution of these variables as the general population . However, given the over-representation of females and those with higher education levels, the data are not fully representative for the Norwegian general population. While not all EQ-5D-5L studies included a comparison with the general population, those that have also found over representation in relation to females , younger  and older age groups , and higher education levels [2, 14, 23, 25]. This may be problematic for the aggregated columns of all respondents, but users can easily weight the data to their own needs. The population norms will be made freely available to Norwegian users for comparative purposes after matching for background characteristics including age and sex-specific strata  or following regression analysis .