  We discuss possible sources of biases in the determination of the initial
mass function (IMF) introduced by the binning of the data, the uncertainty in
the determinations of masses, and the existence of unresolved multiple systems.
Those three effects tend to produce IMFs that are flatter than the real one. We
analyze the importance of each effect and suggest techniques that minimize or
eliminate the biases. We also report the detection of the first astrometric
binary system composed of two very-early O-type stars, HD 93129 A.
