  A thorough understanding of the issues surrounding the determination of
parton distributions is crucial due to their importance to calculations of LHC
observables. However, it is still not fully understood how much of an impact
methodological bias has on PDF fits. Closure tests, where a fit is performed to
pseudo-data generated using an existing PDF set, provide a way of directly
investigating whether current PDF fitting methodologies are successful. Here,
we present a sample of results from closure tests applying the NNPDF
methodology to data created using a variety of different PDF sets. The results
validate our methodology by showing that the initial PDFs can be reproduced
within uncertainties. We also briefly discuss our latest PDF determination,
NNPDF3.0, which has been developed making extensive use of the closure test
technique.
