We have studied x-ray photoelectron diffraction (XPD) for Ag 3d emission from the W(001)c(2×2)-Ag surface with both full multiple-scattering cluster (MSC) and single-scattering cluster analyses. Quantitative structural parameters are obtained for the top layer of the Ag-W alloy structure. Systematic reliability factor searches based on MSC have shown that the outermost layer of the W(001)c(2×2)-Ag surface forms a rumpled Ag-W alloy structure, and the plane containing Ag atoms is 0.3±0.1 higher than the tungsten top layer. It has also been found that, in this system, there is a strong multiple-scattering effect in the photoelectron diffraction even for a high kinetic energy of 1115 eV, where single-scattering theory was generally assumed to be valid. This indicates that, for high Z and closely packed systems, the kinematical method is not suitable for the quantitative analysis of XPD experiments.