Fig. 2.

Model comparison of main dose–response meta-analysis. The solid black line indicates the linear model (a), the quadratic model (b), and the restricted cubic spline model (c). Dashed black lines are 95% point-wise confidence intervals estimated by the respective 1-stage random-effects model. The Akaike Information Criterion values for each model are (a) -3.21, (b) 36.56 and (c) 15.81