Figure 3.

Exposure–response curve of NDVI with depression incidence among SAGE participants in China ($n=\mathrm{8,481}$). Adjustment for sex, age, marital status, urbanicity, education, annual family income, and region. A time-dependent Cox proportional hazards regression model with natural spline function ($\text{df}=3$) was used to analyze the nonlinear relationship. Likelihood ratio test was used to test the nonlinearity. The solid line is the HR, the dashed lines are the 95% CI of the HR, and the green shaded area represents the population density. (A) NDVI in $100\text{-m}$ buffer. (B) NDVI in $250\text{-m}$ buffer. (C) NDVI in $500\text{-m}$ buffer. (D) NDVI in $\mathrm{1,000}\text{-m}$ buffer. Note: CI, confidence interval; df, degrees of freedom; HR, Hazard ratio; NDVI, normalized difference vegetation index; SAGE, Study on Global Ageing and Adult Health.