Data set diagonalization in a global fit

The analysis of data sometimes requires fitting many free parameters in a theory to a large number of data points. Questions naturally arise about the compatibility of specific subsets of the data, such as those from a particular experiment or those based on a particular technique, with the rest of the data. Questions also arise about which theory parameters are determined by specific subsets of the data. I present a method to answer both of these kinds of questions. The method is illustrated by applications to recent work on measuring parton distribution functions.