Variability amplitude, VA, as a function of timescale (left column) and rotational phase difference (right column), for CTTS, WTTS, Class II, and Class III stars (see legends within panels), and for three different X-ray energy bands: full (0.5–8.0 keV, top row), soft (0.5–1.5 keV, middle), and hard (1.5–8.0 keV, bottom). As described in Sect. 3, the VA for each sample and x-axis value (Δt or Δφ) is defined as the 50% width of the distribution of log F₁/F₂ (cf. Fig. 2), i.e. of the difference between the 75% and 25% quantiles. Formal error bars reflect photon statistics and are estimated thorough Monte Carlo simulations. Thick lines refer to the VAs derived from the data, while thin lines with error bars and matching color indicate the contribution of Poisson noise (also estimated with Monte Carlo simulations).