Relative amplitude p of the mean quadratic magnetic field variations against that of the mean magnetic field modulus variations (q). Open symbols: stars showing ⟨ B_z ⟩ reversal; filled symbols: stars in which ⟨ B_z ⟩ has a constant sign. Among the latter, triangles are used to distinguish those stars in which the phase of maximum ⟨ B ⟩ coincides approximately with the phase of lowest absolute value of ⟨ B_z ⟩, while dots are used when ⟨ B ⟩ maximum occurs close to the phase of highest absolute value of ⟨ B_z ⟩.