  In both QCD and supersymmetric QCD (SQCD) with N_f flavors there are
conformal windows where the theory is asymptotically free in the ultraviolet
while the infrared physics is governed by a non-trivial fixed-point. In SQCD,
the lower N_f boundary of the conformal window, below which the theory is
confining is well understood thanks to duality. In QCD there is just a
sufficient condition for confinement based on superconvergence. Studying the
Banks-Zaks expansion and analyzing the conditions for the perturbative coupling
to have a causal analyticity structure, it is shown that the infrared
fixed-point in QCD is perturbative in the entire conformal window. This finding
suggests that there can be no analog of duality in QCD. On the other hand in
SQCD the infrared region is found to be strongly coupled in the lower part of
the conformal window, in agreement with duality. Nevertheless, we show that it
is possible to interpolate between the Banks-Zaks expansions in the electric
and magnetic theories, for quantities that can be calculated perturbatively in
both. This interpolation is explicitly demonstrated for the critical exponent
that controls the rate at which a generic physical quantity approaches the
fixed-point.
