Null hypothesis probability of the chi-square test for the constant model on the normalized Q (red) and U (blue) Stokes parameters for different time (top panel) and energy (bottom panel) binning cases. The left and right vertical axes in each panel show the percentage of the probability values on a logarithmic and linear scale, respectively. The shaded green and orange areas indicate the 1% threshold level. The dashed and dotted black lines in the middle of each panel represent the 1% and 3σ (99.73%) probability levels, respectively.