Wavelet (black dots) and structure function (red squares) timescales calculated during the outburst phase of S4 0954+658 are indicative of multiple variability contributions. In the left panel, the variability is interpreted as the superposition of a source-extrinsic effect (blue line) and a source-intrinsic process (green line). In the right panel, both the variability contributions are attributed to ISS and the two annual modulation patterns have the same scattering screen parameters, the only difference being the size of the scintillating components (90 μas for the blue line, 30 μas for the green line).