  We have compared the oxygen and nitrogen abundances derived from global
emission-line SDSS spectra of galaxies using (1) the Te method and (2) two
recent strong line calibrations: the ON and NS calibrations. Using the Te
method, anomously high N/O abundances ratios have been found in some SDSS
galaxies. To investigate this, we have Monte Carlo simulated the global spectra
of composite nebulae by a mix of spectra of individual components, based on
spectra of well-studied HII regions in nearby galaxies. We found that the Te
method results in an underestimated oxygen abundance (and hence in an
overestimated nitrogen-to-oxygen ratio) if HII regions with different physical
properties contribute to the global spectrum of composite nebulae. This effect
is somewhat similar to the small-scale temperature fluctuations in HII regions
discussed by Peimbert. Our work thus suggests that the high Te-based N/O
abundances ratios found in SDSS galaxies may not be real. However, such an
effect is not expected to be present in dwarf galaxies since they have
generally an uniform chemical composition. The ON and NS calibrations give O
and N abundances in composite nebulae which agree with the mean
luminosity-weighted abundances of their components to within 0.2 dex.
