Natural Experiment

Hong Kong is facing a double-ageing challenge: the population is getting older, so are the buildings. Older people aged 65 or above were 17.9% in 2018 and are expected to increase to 31.9% in 2038; buildings aged over 50 years were 10,200 in 2018 and are estimated to increase 600 per year, reaching half of the city in 2046. Urban Renewal Authority (URA) was established in 2001 to deal with building ageing issues, such buildings are mainly in old urban neighbourhoods, where the ageing population is also concentrated.

Public transport use may influence the physical activity, social inclusion and wellbeing of the elderly. However, previous studies on public transport use and elderly health were mostly observational studies using cross-sectional data. Few studies have examined the before-and-after effects of a new metro, for example, to see if it leads to improved elderly health.

There is a growing interest in using natural experiments for causal inference, considering how built environment changes brought by urban planning and design practices can be structured as health interventions. Without randomisation in intervention exposure as a randomised control experiment does, the challenge of natural experiments for inferring causality lies in validating approximate random distribution of confounding effects between treatment and control groups.