  The CTEQ and MRS parton distributions involve a substantial number (~30) of
parameters that are fit to a large number (~900) of data. Typically, these
groups produce fits that represent a good fit to the data, but there is no
substantial attempt to determine the errors associated with the fits.
Determination of errors would involve consideration of the experimental
statistical and systematic errors and also the errors in the theoretical
formulas that relate the measured cross sections to parton distributions. We
discuss the principles that would be needed in such an error analysis. These
principles are standard. However, certain aspects of the principles appear
counter-intuitive in the case of a large number of data. Accordingly, we strive
to devote careful attention to the logic behind the methods.
