In this paper, the correlation between the local strain distributions and the average strains obtained by an x-ray measurement are studied through numerical modelling. The finite element method is used to simulate the local strain distributions within a three-dimensional random aggregate of isotropic and orthotropic single crystals under uniaxial elastic tension. Calculations are performed for polycrystalline Fe and Cu slabs. The strain/stress values in the crystallites that satisfy the diffraction condition are then used to simulate the x-ray data that would be obtained if the diffraction peaks from such aggregates were measured. In particular the 200, 310, 420, 211 and 222 reflections are analysed. The correlation between the applied far-field stress, the local stresses and the average stresses as determined through x-ray analysis are presented.