How precisely can the difference method determine the pion-nucleon   coupling constant?
  The Coulomb-like backward peak of the neutron-proton scattering differential cross section is due to one-pion exchange. Extrapolation to the pion pole of precise data should allow to obtain the value of the charged pion-nucleon coupling constant. This was classically attempted by the use of a smooth physical function, the Chew function, built from the cross section. To improve accuracy of such an extrapolation one has introduced a difference method. It consists of extrapolating the difference between the Chew function based on experimental data and that built from a model where the pion-nucleon coupling is exactly known. Here we cross-check to which precision can work this novel extrapolation method by applying it to differences between models and between data and models. With good reference models and for the 162 MeV neutron-proton Uppsala single energy precise data with a normalisation error of 2.3 %, the value of the charged pion-nucleon coupling constant is obtained with an accuracy close to 1.8 %. 
B. Loiseau, T. E. O. Ericson