Cardboard floor: about the barriers for social progression and their impact on the representativeness of epidemiological studies.

A continuum of socioeconomic status ranging from the least to the most privileged persons is evidenced in population studies, with profound implications for health and care.1 Individuals in the most disadvantaged social group suffer from extreme poverty and face several specific challenges to their health and healthcare.2 They frequently cannot meet their most basic needs (including their physiological needs, most acutely exemplified by homelessness) and are at a higher risk of health problems and accelerated ageing due to unhealthy habits (eg, unhealthy diet and drug consumption), harmful environmental and biological factors and social isolation.1–4 As a result, the most socially disadvantaged persons have higher rates of premature mortality, especially caused by suicide and violence, and higher prevalence of all types of diseases, particularly infectious diseases and mental disorders.2 5 Besides, care for chronic conditions is compromised for this population group, which relies to a substantial degree in emergency care, particularly in health systems that do not guarantee universal health coverage.5 Even considering the relative size of the most deprived extreme of the social continuum (eg, about 0.5% of the UK adult population in 2018 was considered homeless),6 the scale of …