  We discuss a test of the generalization power of the methodology used in the
determination of parton distribution functions (PDFs). The "future test" checks
whether the uncertainty on PDFs, in regions in which they are not constrained
by current data, are compatible with future data. The test is performed by
using the current optimized methodology for PDF determination, but with a
limited dataset, as available in the past, and by checking whether results are
compatible within uncertainty with the result found using a current more
extensive dataset. We use the future test to assess the generalization power of
the NNPDF4.0 unpolarized PDF and the NNPDFpol1.1 polarized PDF methodology.
Specifically, we investigate whether the former would predict the rise of the
unpolarized proton structure function $F_2$ at small $x$ using only pre HERA
data, and whether the latter would predict the so-called "proton spin crisis"
using only pre-EMC data.
