Observed root-mean-square longitudinal field against the observed average of the mean magnetic field modulus. Open symbols are used to distinguish those stars for which either or both of ⟨ B_z ⟩ _rms and ⟨ B ⟩ _av were computed using less than 7 individual measurements of, resp., ⟨ B_z ⟩ and ⟨ B ⟩; triangles are used for field modulus values from the literature. The dashed line corresponds to ⟨ B_z ⟩ _rms = ⟨ B ⟩ _av/ 3. For only two stars (identified in the figure), the representative points are clearly above this line. However this is significant only for HD 215441, since the number of measurements of HD 157751 is insufficient (see text). The dotted vertical line corresponds to ⟨ B ⟩ _av = 2.8 kG; it emphasises the discontinuity at the low end of the distribution of ⟨ B ⟩ _av, discussed in Sect. 5.1.