  We have compared Monte Carlo photoionization models of H II regions with a
uniform density distribution with models with the same central stars and
chemical compositions but with 3-D hierarchical clumps. We compare the
abundances of He, N, O, Ne, and S obtained from emission line strengths and [O
III] and [N II] temperatures to those in our models. We consider stellar
temperatures in the range 37.5 -- 45kK and ionizing luminosities from 10^{48}
to 10^{51} photons/s.
  Clumped models have different ionic abundances than uniform. For hot stars,
He^0/He^+ is 2 -- 3%, much larger than with uniform models. This amount of He I
is independent of metallicity and so impacts the determination of the
primordial abundance of He. The total abundances of O, Ne, and S obtained by
the usual methods of analysis, using T([OIII) for high stages of ionization and
T([NII]) for low, are about as accurate for clumped models as for uniform and
within about 20% of the true values. If T([OIII]) is used for analyzing all
ions, the derived (O/H) is 40 to 60% too large for cool stars but is good for
hot stars. Uniform models have similar errors, so the clumping does not change
the accuracy of abundance analysis.
  The physical causes of the ionic abundance errors are present in real
nebulae. In clumped models, helium ionizing radiation from zones of high
ionization (low He^0 and low UV opacity) can penetrate nearby regions near the
edge of the ionized zone. This effect allows He^0 to absorb more stellar
photons than in uniform or radially symmetrical geometries. In turn, these
absorptions compete with O+, etc., for those energetic stellar photons.
