Upper panel: histogram showing the distribution of Δ log R_X, defined as the difference between the log of the observed R_X and the log of the R_X from our fit formula. The black line shows a normal distribution with μ = 0 and σ = 0.359 and the red line shows the generalised normal distribution with a probability density function given by βexp[−(|Δlog R_X−μ|/α)^β]/[2αΓ(1/β)], where β = 1.43, μ = 0, α = 0.4, and Γ is the gamma-function. Lower panel: cumulative distribution functions for the variability amplitudes of several samples of stellar L_X measurements. The black line represents the spread in the Ro–R_X distribution, the dotted line represents variability of the Sun, and the other lines show variability distributions derived from the literature as described in Sect. 3.2.