1. A method of extracting systematic defects, the method comprising: inspecting a wafer outside a process window to obtain first inspection data; inspecting the wafer inside the process window to obtain second inspection data; determining an intersection of defects from the first and second inspection data; checking whether the intersection of defects is above or below a threshold count of defects; when the intersection of defects is not below the threshold count of defects, performing the steps of: defining a defect pattern from the first inspection data; and filtering nuisance defects from design data using a pattern search for the defined defect pattern within the design data; inspecting defects inside the process window with greater sensitivity than outside the process window; and determining systematic defects inside the process window.