  We present a NLO perturbative analysis of all available data on the polarized
structure function g_1(x,Q^2) with the aim of making a quantitative test of the
validity of the Bjorken sum rule, of measuring \alpha_s, and of deriving
helicity fractions. We take particular care over the small x extrapolation,
since it is now known that Regge behaviour is unreliable at perturbative
scales. For fixed \alpha_s we find that if all the most recent data are
included g_A=1.18\pm0.09, confirming the Bjorken sum rule at the 8% level. We
further show that the value of \alpha_s is now reasonably well constrained by
scaling violations in the structure function data, despite the fact that it
cannot yet be reliably fixed by the value of the Bjorken sum: our final result
is \alpha_s(m_Z) = 0.120+0.010-0.008. We also confirm earlier indications of a
sizeable positive gluon polarization in the nucleon.
