  The phase diagram of QCD at finite temperature and density and the existence
of a critical point are currently very actively researched topics. Although
tremendous progress has been made, in the case of two light quark flavors even
the order of the phase transition at zero density is still under discussion.
Finite-size scaling is a powerful method for the analysis of phase transitions
in lattice QCD simulations. From the scaling behavior, critical exponents can
be tested and the order as well as the universality class of a phase transition
can be established. This requires knowledge of the critical exponents and the
scaling behavior. We use a non-perturbative Renormalization Group method to
obtain critical exponents and the finite-size scaling functions for the O(4)
universality class in three dimensions. These results are useful for a
comparison to the actual scaling behavior in lattice QCD simulations with two
flavors, as well as for an estimate of the size of the scaling region and the
deviations from the expected scaling behavior.
