Industrialised countries are experiencing significant demographic changes as people live longer and have healthier life and this is particularly true in Europe. Demographic projections by Eurostat (2011) indicate that the old-age dependency ratio - the ratio of those outside the labour force to those of working age (15–64 yr)- will double from 25.9% on average (in the 27 European countries) in 2010 to 50.2% by 2050.By then, two people of working age will be needed to support one pensioner. This rapidly ageing population is thus presenting challenges to the age structure of the workforce and to the sustainability of social protection schemes. As a result, national authorities in most European countries are trying to promote work ability throughout the working life, considering changes in the legal age for retirement, and preparing their citizens to a world in which everybody will stay longer on the labour market.
In this context, it is of utmost importance to identify simple ways to monitor work ability in the working population on a regular basis. The Finnish Institute of Occupational Health (FIOH) played a pioneering role during the 1980’s when it developed a generic tool to assess work ability, the so-called “Work Ability Index” (WAI); it considers the workers’ self-assessed work ability in relation to work requirements, health status and the worker resources . WAI has since then been widely disseminated and is nowadays the most commonly used tool for measuring work ability .
It has been shown in several studies performed in various professional groups that age, obesity, lack of physical activity during spare time, low musculoskeletal ability, high mental requirements, lack of autonomy and heavy physical workload, all have a negative impact on the WAI level . Other studies have demonstrated that WAI also has a predictive value, a low WAI level (or a level declining over time) increasing the probability of sick leaves [3–5], of early retirement [6–8] and even of worker decease .
Taking into consideration the WAI predictive validity and striving to improve the employment rate among workers aged over 55, the largest occupational health service in Luxembourg - called “Service de Santé au Travail Multisectoriel” (STM) - decided as from 2005 to use WAI within the framework of regular monitoring of workers’ health; this enabled the creation of a substantial database of WAI levels concerning its affiliated workers.
Based on the analysis of this database, the present study aims to answer the following questions:
1) Could the assessment of work ability be based on the use of a single item of WAI, the first one, which consists in self-assessment by workers of their current work ability level by comparison with the highest work ability experienced during their career, this question being termed “Work Ability score” (WAS) by the designers of the method ? Assessing work ability from this single question appears seducing on the grounds that the assessor has not to check the categorisation of the job function reported by the worker and it is also more understandable by the persons surveyed; some other WAI items, the 3rd one - asking for the number of diagnosed diseases or the 7th one – assessing psychological resources – are for instance often not well understood and may be left unanswered [10–15]. In addition, the 1st item of WAI has high discriminating power (the highest) over the entire index . This measure would in theory be easier both to implement and interpret in population surveys and could be carried out at a lower cost [13, 17]. However, before possibly adopting this simplified procedure, one needs to assess its validity when compared to the full WAI. To date, such an analysis has only been performed in a relatively limited population of long-term disabled workers , in a sample of the general population in Finland  and in a group of Dutch construction workers .
2) Are age, body mass index, daily smoking, health status, firm size and type of work function determinants of the WAI level?
The answers to these two research questions would provide useful information when drawing guidelines for the national policy of promotion of active ageing which the Luxembourg government wishes to put in place.