Abstract: A method for inspecting a repeating pattern of a sample for defects. The repeating pattern has a translation vector, which defines a separation of substantially identical sections of the repeating pattern. The repeating pattern has a first section and a second section which are separated substantially by the translation vector. The method includes comparing the first section and the second section to identify a discrepancy indicative of a possible defect. The method also includes analyzing a reference data source in order to identify if the defect exists, and when the defect exists, in which one of the first section and the second section the defect exists. The reference data source is derived from a source exogenous to the sample.