Directional semivariograms for breast and cervix cancer mortality rates with the anisotropic model fitted. The graphs for breast cancer (left column) were created by poisson_kriging.exe using the parameter file of Figure 3 with the option ndir = 4. The estimators are the followings: traditional semivariogram (Equation 8 with n(uα) = 1 ∀ α), population-weighted semivariogram (Equation 8), and risk semivariogram (Equation 18). The semivariograms are computed in four directions; azimuth angles are measured in degrees clockwise from the NS axis The solid curve denotes the anisotropic (i.e. direction-dependent) model fitted using weighted least-square regression.