Distributions of the spectral index calculated from the NVSS/GB6 sample for different flux density intervals: i₁) S = [100, 158] mJy; i₂) S = [158, 251] mJy; i₃) S = [251, 400] mJy; i₄) S ≥ 400 mJy. The thick black lines are the “best-fit” truncated Gaussians, and the thin red lines are the total distributions obtained from them after introducing source variability (see the text).