  The frequency distributions of sizes of ``clouds" and ``clumps" within clouds
are significantly flatter for extinction surveys than for CO spectral line
surveys, even for comparable size ranges. A possible explanation is the
blocking of extinction clouds by larger foreground clouds (occlusion), which
should not affect spectral line surveys much because clouds are resolved in
velocity space along a given line of sight. We present a simple derivation of
the relation between the true and occluded size distributions, assuming clouds
are uniformly distributed in space or the distance to a cloud comples is much
greater than the size of the complex. Because the occlusion is dominated by the
largest clouds, we find that occlusion does not affect the measured size
distribution except for sizes comparable to the largest size, implying that
occlusion is not responsible for the discrepancy if the range in sizes of the
samples is large. However, we find that the range in sizes for many of the
published observed samples is actually quite small, which suggests that
occlusion does affect the extinction sample and/or that the discrepancy could
arise from the different operational definitions and selection effects involved
in the two samples. Size and mass spectra from an IRAS survey (Wood \etal\
1994) suggest that selection effects play a major role in all the surveys. We
conclude that a reliable determination of the ``true" size and mass spectra of
clouds will require spectral line surveys with very high signal-to-noise and
sufficient resolution and sampling to cover a larger range of linear sizes, as
well as careful attention to selection effects.
