  When comparing experimental results with theoretical predictions of the
Casimir force, the accuracy of the theory is as important as the precision of
experiments. Here we evaluate the Casimir force when finite conductivity of the
reflectors and finite temperature are simultaneously taken into account. We
show that these two corrections are correlated, i.e. that they can not, in
principle, be evaluated separately and simply multiplied. We estimate the
correlation factor which measures the deviation from this common approximation.
We focus our attention on the case of smooth and plane plates with a metallic
optical response modeled by a plasma model.
