In this paper we propose a new method of data analysis to interpret results obtained by Thermally Stimulated Depolarisation Current (TSDC) measurements on glassy materials. We give a new law to link the evolution of the relaxation time τ with the temperature T. While the classical law (compensation law) is only convenient in a small range of temperatures (around the compensation temperature T _c), a calculation of the depolarisation current, taking into account this new law, leads to a curve I vs. T very close to the real depolarisation current measured on a polymer in the whole range of temperatures. In addition this new law allows the determination of the compensation temperature T _c from the complex spectrum which needs single scan of measurements. To our knowledge it is the first time that this determination is proposed from the complex spectrum.