  At large values of $x$ the parton distribution functions (PDFs) of the proton
are poorly constrained and there are considerable variations between different
global fits. Data at such high $x$ have already been published by the ZEUS
Collaboration, but not yet used in PDF extractions. A technique for comparing
predictions based on different PDF sets to the observed number of events in the
ZEUS data is presented. It is applied to compare predictions from the most
commonly used PDFs to published ZEUS data at high Bjorken $x$. A wide variation
is found in the ability of the PDFs to predict the observed results. A scheme
for including the ZEUS high $x$ data in future PDF extractions is discussed.
