  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.
