Number of magnetic massive stars as a function of the logarithm of the dipolar magnetic field strength B_d. The histogram was drawn using a fixed bin size of Δlog (B_d) = 0.2 dex. The histogram drawn with a dashed line was obtained considering only those stars for which a definite value of B_d is available (i.e., excluding the stars for which only a lower limit on B_d is available). The arrows show the position of the four stars with the weakest magnetic field (for ϵ CMa the B_d value is a lower limit). The vertical dotted line indicates the magnetic field detection threshold of the FORS low-resolution spectropolarimeters (see text for more details).