A method of using a chemical array unit having a chemical array with probes at multiple feature locations. A request for test may be read, which test uses a sub-array of the array. A pattern of the sub-array may be retrieved from a memory using the test request, which memory carries a pattern for the sub-array which is retrievable with the different test request. Also, a method of reading a chemical array unit which has been exposed to a sample, and feature locations of which have been rendered incapable of providing signal data representative of binding of a sample component. Further methods, apparatus, and computer program products are provided.