Nebular abundances in galaxies: Beware of biases
  The derivation of nebular abundances in galaxies using strong line methods is
simple and quick. Various indices have been designed and calibrated for this
purpose, and they are widely used. However, abundances derived with such
methods may be significantly biased, if the objects under study have different
structural properties (hardness of the ionizing radiation field, morphology of
the nebulae) than those used to calibrate the methods. Special caution is
required when comparing the metallicities of different samples, like, for
example, blue compact galaxies and other emission line dwarf galaxies, or
samples at different redshifts.
