  A three-dimensional effective theory of Polyakov loops has recently been
derived from Wilson's Yang-Mills lattice action by means of a strong coupling
expansion. It is valid in the confined phase up to the deconfinement phase
transition, for which it predicts the correct order and gives quantitative
estimates for the critical coupling. In this work we study its predictive power
for further observables like correlation functions and the equation of state.
We find that the effective theory correctly reproduces qualitative features and
symmetries of the full theory as the continuum is approached. Regarding
quantitative predictions, we identify two classes of observables by numerical
comparison as well as analytic calculations: correlation functions and their
associated mass scales cannot be described accurately from a truncated
effective theory, due to its inherently non-local nature involving long-range
couplings. On the other hand, phase transitions and bulk thermodynamic
quantities are accurately reproduced by the leading local part of the effective
theory. In particular, the effective theory description is numerically superior
when computing the equation of state at low temperatures or the properties of
the phase transition.
