  We perform the NLO QCD analysis of the world data on inclusive deep inelastic
scattering cross sections of charged leptons off the proton and the deuterium
targets. The parton distributions, the value of strong coupling constant
$\alpha_{\rm s}$, and the twist 4 contributions to the structure functions
$F_2$ and $F_{\rm L}$ are extracted with the complete account for the
correlations of data points due to the systematic errors. Sensitivity of the
$\alpha_{\rm s}$ value and the high twist contribution to the procedures of
accounting for the systematic errors is studied. The impact of theoretical
uncertainties on the value of $\alpha_{\rm s}$ and on the parton distributions
is analysed. The obtained value of strong coupling constant with the account of
these uncertainties is $\alpha_{\rm s}(M_{\rm Z})=0.1165\pm0.0017({\rm
stat+syst}) \pm^{0.0026}_{0.0034}({\rm theor})$. The uncertainties of
parton-parton luminosities for the FNAL and LHC colliders are estimated.
