Stability of the dielectric parameters in the alpha beta splitting region against methodical variations is demonstrated. The Williams product ansatz for correlation functions and an additive ansatz for dielectric functions are compared for poly(ethyl methacrylate). Both evaluation methods give a diminishing a intensity with increasing temperature, and a separate trace for the alpha relaxation different from both the secondary beta relaxation below and the high-temperature a process above the crossover. Furthermore, measurements on different poly(n-butyl methacrylate) samples and evaluation results of different experimentalists are compared. The intensity onset of the cooperative a relaxation, starting from zero at a characteristic temperature T-on in the crossover region, and the parallel course of alpha and beta relaxations below T-on, separated by about one frequency decade in an Arrhenius diagram, remain stable against measurement and evaluation variations.