Xuxi Zhang

psychological and social domains. 22 However, the reliability and validity of the psychological and social domains have otherwise received little attention in research before. To the best of our knowledge, this is the first study to report the reliability and validity of the TFI for multiple European countries simultaneously and the first in Greece, Croatia and the UK. We investigated the validity of the full TFI and its three domains. However, some limitations of our study should be highlighted. First, we did not assess the consistency of the TFI over time (test ‐ retest reliability). However, frailty is not assumed to be stable over time and a low test ‐ retest correlation over the follow up period (12 months) may be expected. Therefore, we believe that assessing the consistency of the TFI across items (internal consistency) is sufficiently adequate for the current study. Second, we did not assess the sociocultural and language differences in the interpretation of individual items between countries. Consequently, we may have observed some unintended variation between countries. Still, we have paid specific attention to translating the items of the TFI for which no validated translation was available (Greece, Croatia). Further studies on the cultural adaption of the items are needed to confirm our findings. Third, most of the alternative measures chosen to examine convergent and divergent validity and concurrent validity have been widely applied by previous studies. However, there is no golden standard of choosing alternative measures of the TFI, and the number of alternative measures for psychological and social domains was limited by the data availability of the UHCE project. Further studies with more alternative measures are still needed. Finally, the application of the TFI in clinical practice still needs further study due to the absence of general population norms or reference scores, 9 and further research on the use of the TFI in other settings such as the hospital setting is still required. CONCLUSIONS AND IMPLICATIONS In summary, our study supported the reliability and validity of the full TFI and physical domain. The TFI may be applied as an instrument to assess frailty in community ‐ dwelling older people for large ‐ scale population studies on frailty in the five European countries. However, our conclusions are drawn from statistical methods, and we cannot prove whether the use of the TFI will lead to clinically meaningful outcomes. The reliability and validity of the psychological and social domains have not been studied extensively before and more investigations in different countries are needed in the future. 4 93 Reliability and validity of the TFI