  X-ray luminosity functions (XLF) are frequently used to assess the properties
of the X-ray emission from a given sample of stars, and particularly important
for a comparison of different groups of stars. I present the results from a
detailed analysis of the XLF of pre-main sequence T Tauri Stars in
Taurus-Auriga and young main-sequence objects (represented by the Pleiades and
Hyades cluster). The study is based on the ROSAT public archive. Various
possible biases related to the sample selection are described, and their
influence on the luminosity distributions is discussed.
