Statistical studies of the resolved stellar populations of massive stars in the
Milky Way and nearby galaxies have helped to constrain stellar evolution models.
For example, the ratio of the number of O-type stars to Wolf-Rayet stars in galaxies
with different metallicities and at different locations within the galaxy can give
important clues about mass-loss (e.g., Maeder & Meynet 1994; Massey 2003).