  Imaging surveys of CO and other molecular transition lines are fundamental to
measuring the large-scale distribution of molecular gas in the Milky Way. Due
to finite angular resolution and sensitivity, however, observational effects
are inevitable in the surveys, but few studies are available on the extent of
uncertainties involved. The purpose of this work is to investigate the
dependence of observations on angular resolution (beam sizes), sensitivity
(noise levels), distances, and molecular tracers. To this end, we use
high-quality CO images of a large-scale region (25.8 <l< 49.7 deg and |b|<5
deg) mapped by the Milky Way Imaging Scroll Painting (MWISP) survey as a
benchmark to simulate observations with larger beam sizes and higher noise
levels, deriving corresponding beam filling and sensitivity clip factors. The
sensitivity clip factor is defined to be the completeness of observed flux.
Taking the entire image as a whole object, we found that 12CO has the largest
beam filling and sensitivity clip factors and C18O has the lowest. For
molecular cloud samples extracted from images, the beam filling factor can be
described by a characteristic size, $l_{1/4}$=0.762 (in beam size), at which
the beam filling factor is approximately 1/4. The sensitivity clip factor shows
a similar relationship but is more correlated with the mean voxel
signal-to-noise ratio of molecular clouds. This result may serve as a practical
reference on beam filling and sensitivity clip factors in further analyses of
the MWISP data and other observations.
