  We determine the strong coupling alpha_s from a next-to-leading order
analysis of processes used for the NNPDF2.1 parton determination, which
includes data from neutral and charged current deep-inelastic scattering,
Drell-Yan and inclusive jet production. We find alpha_s(M_Z)=0.1191+-0.0006
(exp), where the uncertainty includes all statistical and systematic
experimental uncertainties, but not purely theoretical uncertainties. We study
the dependence of the results on the dataset, by providing further
determinations based respectively on deep-inelastic data only, and on HERA data
only. The deep-inelastic fit gives the consistent result
alpha_s(M_Z)=0.1177+-0.0009(exp), but the result of the HERA-only fit is only
marginally consistent. We provide evidence that individual data subsets can
have runaway directions due to poorly determined PDFs, thus suggesting that a
global dataset is necessary for a reliable determination.
