Regression line slopes with error bars for the point populations in diagrams of the type of Fig. 4, based on model fitting with different fixed values for the cancellation exponent κ. The filled circles and solid line represent magnetograms with 5-min integration, while the open circles and dashed line represent magnetograms with 1-min integration. Only zero slope is physically acceptable, which occurs when κ = 0.13 (marked by the vertical dotted line).