Described herein is a method for identifying an aberrant feature on a nucleic acid array. In general terms, the method comprises: a) obtaining a log transformed normalized value indicating the amount of hybridization of a test sample to a first feature on the nucleic acid array; b) calculating a z-score for the first feature using: the log transformed normalized value; and the distribution of reference log transformed normalized values that indicate the amount of hybridization of control samples to the same feature on a plurality of reference arrays; and c) identifying the test feature as aberrant if it has a z-score that is above or below a defined threshold.