Estimating life expectancy and years of life lost in epidemiological studies: a review of methods using an example from multimorbidity

Background and objectiveThere has been an increasing interest in using life expectancy metrics, such as years of life lost (YLL), to explore epidemiological associations. YLL is easier to understand for both healthcare professionals and the lay people and has become a common measure for evaluating public health priorities. As the literature presents a range of approaches to estimate it, this review aims to: (1) summarise the key methods; (2) show how to implement them using current software; (3) apply them in a real-world example. MethodsWe investigated simpler nonparametric as well as parametric, model-based methods to estimate of YLL, including: (1) Years of potential life lost (YPLL); (2) Global Burden of Disease (GBD) approach; (3) Chiangs life tables; (4) Epi-demographic approach; and (5) Flexible Royston-Parmar parametric survival model. We used data from the UK Biobank with baseline measures collected in 2006-2010 and linkage to mortality records. We selected 36 chronic conditions: participants with two or more conditions were categorised as having multimorbidity. ResultsFor the YPLL and GBD method, the analytical procedures allow only to quantify the average YLL within each group (with and without multimorbidity) and, from them, their difference. Conversely, for the Chiangs life tables, the epi-demographic approach, and the Royston-Parmar survival model, both the remaining life expectancy within each group and the YLL could be estimated. In 499,992 UK Biobank participants (white ethnicity, 94%; women, 55%) with a median (IQR) age of 58 (50-63) years, 98,605 (20%) had multimorbidity and 11,871 deaths occurred during the follow-up. The YLLs comparing subjects with vs without multimorbidity varied significantly according to the technique and the modelling approach used: from a longer life expectancy in subjects with multimorbidity using the YPLL and the GBD method to a shorter one using the other three methods (i.e., at 65 years, the YLL were 1.8, 1.3, and 4.6 years using Chiangs, epi-demographic, and Royston-Parmar approach, respectively). ConclusionsWhen comparing the burden of a disease on life expectancy across studies caution is needed as methods may estimate different quantities. While deciding among different methods to estimate YLL, researchers should consider such differences in relation to the purpose of the research and the type of available data. O_TEXTBOXSUMMARY BOXO_LIThe concept of years of life lost (YLL) is easier to understand compared to traditional estimates from survival analysis, such as hazard ratios, but very few studies report it. C_LIO_LIA range of different methods of estimating YLL are reviewed: from basic methods - such as life tables - to most recent and advanced methods using statistical modelling. C_LIO_LIUsing the example of multimorbidity, the estimated numbers of YLL differs between methods, as each method focused on estimating different quantities. C_LIO_LIThis review will help promote a better understanding and use of life expectancy and YLL metrics in a wide range of studies in health care research. C_LI C_TEXTBOX