1. A method for providing an estimate of yield for a device in a fabrication facility, the device having a layout including a pattern characterizable by a combination of members of a set of basis shapes, comprising: providing a set of basis pre-images in a machine readable storage medium, the basis pre-images in the set including yield map data representing an interaction of respective members of the set of basis shapes with a defect model; reading and combining using a data processor, basis pre-images corresponding to basis shapes in said combination of members of said set of basis shapes to provide a combination result; and outputting said combination result.