Xuxi Zhang

INTRODUCTION With the population rapidly ageing worldwide and the increasing prevalence of chronic multi ‐ morbidity, frailty is increasingly recognized as a complex and important public health issue. 1, 2 People with frailty have a higher risk of various negative outcomes such as falls 3 , disability 4 , long ‐ term care 5 , hospitalization 4 and mortality 6 . To improve the management of frailty and deliver more patient ‐ centered care, providing supportive care to people with frailty ideally starts with the identification of their severity level of frailty. 7 Although many assessment tools to measure the severity level of frailty have been developed in the past decades 7, 8 , there is no global standard assessment measure for frailty. 8 Hence, it is important to have robust data and studies on the psychometric properties including reliability and validity of existing instruments, in order to be able to compare and select the most appropriate and relevant health measurement tools. Furthermore, researchers, health care professionals and policymakers increasingly acknowledge the multidimensional nature of frailty. 1, 5, 9 However, most frailty assessment measures only cover the physical domain 4, 10, 11 , but not the psychological and social domains. 9 The Tilburg Frailty Indicator (TFI) is a short self ‐ reported questionnaire, originally developed for identifying frail community ‐ dwelling older people in the Netherlands in 2010. 5, 12 It considers frailty from a bio ‐ psycho ‐ social framework, which includes 15 items addressing 3 domains: the physical, psychological and social domains. 12 Pialoux et al 13 found that the TFI is one of the best three measures for screening frailty in primary health care settings. The psychometric properties of the TFI have been extensively examined especially in Dutch populations. 9, 12, 14 However, the validity of the single domains of the TFI, especially the psychological and social domains, has not yet been extensively examined. 15 ‐ 19 In addition, research on the properties of the TFI among different populations are still lacking. 5 For example, the TFI has not yet been validated in Greece, Croatia or the United Kingdom (UK). Conducting the validation study in these countries contributes to the current literature with important evidence on psychometric properties of the TFI. Furthermore, reporting the results of the total population of the five European countries contributes to the generalizability of the results to other local contexts. This study aims to assess the reliability and validity of the full TFI and its three domains in a population of community ‐ dwelling older people from 5 European countries, including Spain, Greece, Croatia, the Netherlands and the UK. In addition, the reliability and validity will be assessed for each country separately. We examined the following aspects: (1) the internal consistency (reliability) of the full TFI and the 3 domains; (2) the convergent and divergent validity (construct validity) of the 3 domains; and (3) the concurrent validity (criterion validity) of the full TFI and the 3 domains. 4 79 Reliability and validity of the TFI