Gamma-ray bursts (GRBs) are sporadic flashes of light observed primarily in the gamma-ray band. Being the brightest explosions in the Universe since its birth, they rank among the furthest astronomical sources ever detected. Determining the relevant timescales of any astronomical phenomenon is essential to understand its associated physical processes. Linear methods in time-series analysis are powerful tools for the researcher that can provide insight into the underlying dynamics of the studied systems. When these methods were used in the study of GRBs with known redshift from a small mixed sample of BATSE-Konus-BeppoSAX observations, correcting for the first time for cosmic time dilation effects, the existence of two classes of variability was revealed. However, the origin of these classes remains unclear. Preliminary results based on the analysis of Swift bursts will be presented and compared with previous findings.