The estimated standard deviation for the mean radius of structural elongation of the sample. In a given solute-solvent contrast, the radius of gyration R_G is a measure of structural elongation if the internal inhomogeneity of scattering densities has no effect. Guiner analysis at low Q give the R_G and the forward scattering at zero angle I(0). lnl(Q) = lnl(0) - R_G^2Q2/3 where Q = 4(pi)sin(theta/lamda) 2theta = scattering angle lamda = wavelength The above expression is valid in a QR_G range for extended rod-like particles. The relative I(0)/c values ( where c = sample concentration) for sample measurements in a constant buffer for a single sample data session, gives the relative masses of the protein(s) studied when referenced against a standard. see: O.Glatter & O.Kratky, (1982). Editors of "Small angle X-ray Scattering, Academic Press, New York. O.Kratky. (1963). X-ray small angle scattering with substances of biological interest in diluted solutions. Prog. Biophys. Chem., 13, 105-173. G.D.Wignall & F.S.Bates, (1987). The small-angle approximation of X-ray and neutron scatter from rigid rods of non-uniform cross section and finite length. J.Appl. Crystallog., 18, 452-460. If the structure is elongated, the mean radius of gyration of the cross-sectional structure R_XS and the mean cross sectional intensity at zero angle [I(Q).Q]_Q->0 is obtained from ln[I(Q).Q] = ln[l(Q).(Q)]_Q->0 - ((R_XS)^2Q2)/2