A method of testing a wafer after a current top layer is formed over the wafer. Stress data is collected for the wafer after forming the current top layer. The stress data is derived from changes in wafer curvature. The stress data includes: stress-xx in an x direction and stress-yy in a y direction for each area of a set of finite areas on the wafer, the stress-xx and stress-yy both being derived from wafer-curvature-change-xx in the x direction for each area of the set of finite areas and from wafer-curvature-change-yy in the y direction for each area of the set of finite areas; and the stress-xy being derived from wafer-curvature-change-xy, wherein wafer-curvature-change-xy is a change in wafer twist in the x-y plane for each area of the set of finite areas. A stress gradient vector (and/or its norm) is calculated and used to evaluate the investigating single or multiple accumulated layer.