  We examine the biases inherent to chemical abundance distributions when
targets are selected from the red giant branch (RGB), using simulated giant
branches created from isochrones. We find that even when stars are chosen from
the entire colour range of RGB stars and over a broad range of magnitudes, the
relative numbers of stars of different ages and metallicities, integrated over
all stellar types, are not accurately represented in the giant branch sample.
The result is that metallicity distribution functions derived from RGB star
samples require a correction before they can be fit by chemical evolution
models. We derive simple correction factors for over- and under-represented
populations for the limiting cases of single-age populations with a broad range
of metallicities and of continuous star formation at constant metallicity; an
important general conclusion is that intermediate-age populations (~1-4Gyr) are
over-represented in RGB samples. We apply our models to the case of the Large
Magellanic Cloud bar and show that the observed metallicity distribution
underestimates the true number of metal-poor stars by more than 25%; as a
result, the inferred importance of gas flows in chemical evolution models could
potentially be overestimated. The age- and metallicity-dependences of RGB
lifetimes require careful modelling if they are not to lead to spurious
conclusions about the chemical enrichment history of galaxies.
