  The recently developed "Data Set Diagonalization" method (DSD) is applied to
measure compatibility of the data sets that are used to determine parton
distribution functions (PDFs). Discrepancies among the experiments are found to
be somewhat larger than is predicted by propagating the published experimental
errors according to Gaussian statistics. The results support a tolerance
criterion of $\Delta\chi^2 \approx 10$ to estimate the 90% confidence range for
PDF uncertainties. No basis is found in the data sets for the much larger
$\Delta\chi^2$ values that are in current use; though it will be necessary to
retain those larger values until improved methods can be developed to take
account of systematic errors in applying the theory. The DSD method also
measures how much influence each experiment has on the global fit, and
identifies experiments that show significant tension with respect to the
others. The method is used to explore the contribution from muon scattering
experiments, which are found to exhibit the largest discrepancies in the
current fit.
