  We apply the method of principal component analysis to a sample of simple
stellar populations to select some age-sensitive spectral indices. Besides the
well-known age-sensitive index, H B, we find that some other spectral indices
have great potential to determine the age of stellar populations, such as
G4300, Fe4383, C24668, and Mgb. In addition, we find that the sensitivity to
age of these spectral indices depends on the metallicity of the simple stellar
population (SSP): H B and G4300 are more suited to determine the age of the
low-metallicity stellar population, C24668 and Mgb are more suited to the
high-metallicity stellar population. The results suggest that the principal
component analysis method provides a more objective and informative alternative
to diagnostics by individual spectral lines.
