  Collective phenomena from RHIC and LHC experiments indicate a strongly
coupled quark gluon plasma. Finite temperature lattice QCD calculations show
the interaction measure, $\Delta\equiv(\epsilon-3p)/T^4$, is sizeable over a
considerable range of temperatures above the deconfinement temperature, which
may also indicate that the plasma is strongly interacting. For the ideal gas
with massless particles, the interaction measure is zero. A nonzero value is
probably due to either the interaction or the mass. In order to see the
contribution of the particle mass to the interaction measure, in this paper we
study a system without any interactions, i.e., an ideal gas with massive
particles. After assembling the standard formulas of the quantum statistics
with relativistic energy, we calculate the energy density, the pressure and the
interaction measure. We find that their dependences on temperature reproduce
the qualitative features of the lattice result. The interaction measure is
nonzero for an ideal gas, which demonstrates that the particle mass contributes
to the interaction measure. By our estimate, in the interaction measure
obtained by the lattice calculation, quark mass contributes less than (40-50)%.
There are sizeable residue interactions in the deconfined phase.
