OBJECTIVES: Health economic modelling is fundamental to assessing the cost-effectiveness of new interventions and determining the efficient allocation of healthcare resources. Health economic models often consider the patient population to be a single cohort with fixed average patient demographics as a simplifying assumption. However, demographics such as sex can have a significant impact on patient outcomes and so average demographics may change over time. The objective of this study was to quantify the impact of modelling survival stratified by sex in comparison with a typical approach assuming fixed average patient demographics.

METHODS: A lifetime model was developed to assess population outcomes in terms of life years (LY) and quality-adjusted life years (QALY) gained. The modelled population were assumed to be 18 years old at baseline, and survival was modelled using UK life table estimates combined with published sex specific EQ-5D population norms. Two scenarios were assessed, one with a single cohort with a fixed distribution of sex (50% male, 50% female) with a weighted average utility and mortality risk, and one where outcomes for each sex were assessed independently, and then aggregated.

RESULTS: Treating the modelled population as a single homogenous cohort with fixed average baseline characteristics resulted in a total LY gain of 62.21, translating to a QALY gain of 51.12 over a lifetime horizon. In comparison, modelling outcomes for males and females independently resulted in LY and QALY gains of 62.33 and 51.16, respectively. Consequently, modelling patients as a single homogeneous cohort resulted in 0.11 and 0.05 fewer LYs and QALYs gained, respectively. This corresponds to a value of £1,385 at a willingness-to-pay threshold of £30,000/QALY.

CONCLUSIONS: Modelling patient populations using mean demographic characteristics may systematically underestimate total benefits. As a result, modelling interventions with the potential to extend life expectancy with this assumption may underestimate incremental benefits, and consequently cost-effectiveness.