  I explore the effects of observational errors on nebular chemical abundances
using a sample of 179 optical spectra of 42 planetary nebulae (PNe) observed by
different authors. The spectra are analysed in a homogeneous way to derive
physical conditions and ionic and total abundances. The effects of
recombination on the [O II] and [N II] emission lines are estimated by
including the effective recombination coefficients in the statistical
equilibrium equations that are solved for O$^+$ and N$^+$. The results are
shown to be significantly different than those derived using previous
approaches. The O$^+$ abundances derived with the blue and red lines of [O II]
differ by up to a factor of 6, indicating that the relative intensities of
lines widely separated in wavelength can be highly uncertain. In fact, the He I
lines in the range 4000-6800 A imply that most of the spectra are bluer than
expected. Scores are assigned to the spectra using different criteria and the
spectrum with the highest score for each PN is taken as the reference spectrum.
The differences between the abundances derived with the reference spectrum and
those derived with the other spectra available for each object are used to
estimate the one-sigma observational uncertainties in the final abundances:
0.11 dex for O/H and Ar/H, 0.14 dex for N/H, Ne/H, and Cl/H, and 0.16 dex for
S/H.
