Thirteen-month running means applied to the solar radii obtained from the Grueff maps (18.3 GHz). The red points indicate R_c, the blue points indicate R_eq, the green points indicate R_pol, and the black points indicate the sunspot index number. Panels a and b refer to the HP method and IP method, respectively. Panels c and d refer to the HP method and IP method, respectively, for the ellipsis-based statistical procedure. Panels e and f refer to the HP method and IP method, respectively, for the circle-based statistical procedure. Panels g and h refer to the ECB and 2GECB models, respectively.