  A star formation efficiency per free fall time that evolves over the life
time of giant molecular clouds (GMCs) may have important implications for
models of supersonic turbulence in molecular clouds or for the relation between
star formation rate and H2 surface density. We discuss observational data that
could be interpreted as evidence of such a time variability. In particular, we
investigate a recent claim based on measurements of H2 and stellar masses in
individual GMCs. We show that this claim depends crucially on the assumption
that H2 masses do not evolve over the life times of GMCs. We exemplify our
findings with a simple toy model that uses a constant star formation efficiency
and, yet, is able to explain the observational data.
