The Slope of the Upper End of the IMF and the Upper Mass Limit: An
  Observer's Perspective
  There are various ways of measuring the slope of the upper end of the IMF.
Arguably the most direct of these is to place stars on the H-R diagram and
compare their positions with stellar evolutionary models. Even so, the masses
one infers from this depend upon the exact methodology used. I briefly
discusssome of the caveats and go through a brief error analysis. I conclude
that the current data suggest that the IMF slopes are the same to within the
errors. Similarly the determination of the upper mass "limit" is dependent upon
how well one can determine the masses of the most massive stars within a
cluster. The recent finding by Crowther et al (2010) invalidates the claim that
there is a 150Mo upper limit to the IMF, but this is really not surprising
given the weakness of the previous evidence.
