Mechanical properties of viscoelastic materials are modeled, in time domain, with creep compliance and relaxation modulus, depending on a type of excitation. The creep compliance and the relaxation modulus are defined as a response of a material to the instantaneously applied constant load, mathematically modeled as a step function. In reality such loading can not be accomplished, therefore it is substituted with the load in the shape of a ramp function. This substitution vitiate the quality of measurements. This paper presents an analysis of the introduced error and possibility to optimize the measuring procedure according to the pre-defined error.