Recent calculations of the plasma sheet polytropic index based on different data sets resulted in quantitatively different conclusions. Baumjohann and Paschmann [1989> (referred to as BP) used AMPTE/IRM data, obtaining an index around 5/3. Huang et al. [1989> (Referred to as HCF) used ISEE LEPEDEA data, obtaining an index less than 1. Using the ISEE FPE ion data, we demonstrate in this work that 1. The above noted difference is due primarily to the different variables the authors used to estimate the index; 2. the estimated index varies with the degree of the data scattering; 3. the polytropic index of the central plasma sheet is found to be larger than 1 but less than 1.67; 4. the variation of the specific entropy among flux tubes is considerable and indicates the different origins of the plasma in different flux tubes. ¿ American Geophysical Union 1990