Sample of β_i from H.E.S.S.  data on Vela X  (Abramowski et al. 2012) for ΔE = 1 to 1.3 TeV integrated over the whole zenith-angle and offset range. Top: sample of corrected hadrons β after interpolation and extrapolation. The respective value β_eff is marked by the line. Middle: sample of relative errors of the above samples. Here, the black line indicates the median relative error used to calculate σ(β_eff). Bottom: sample of the errors of the above β sample. Marked is the value of σ(β_eff) which is estimated with the help of the median relative error indicated as a black line in the middle plot. See text for further information.