Issues in alpha_s variation
The variation induced by the change of the renormalization scale for αs in PDF is not symmetric and should not be symmetrized artificially, as done here. Moreover, the weights stored in Summer16
samples correspond to a variation of ±1×10-3, while PDF4LHC15 recommend a larger variation of ±1.5×10-3. The up and down deviations should therefore be rescaled by factor 1.5.
Edited by Andrey Popov