A novel analytical approach has been developed to assess the plasticity induced error in residual stress (RS) distributions measured by the contour method (CM) for which experimental validation is required. The aim of the proposed experiment is to quantify the state of residual stress in a specially designed laser welded plates made from ferritic steel (AISI Grade 1005) using neutron diffraction. Residual stresses in the same plates will then be measured by the CM and compared with the neutron results and measures of stress error determined in order to validate the new CM error prediction approach.