1. A method for isolating the location of defects in a scan chain, comprising: performing diagnostic fault simulation on a plurality of scan chain faults; gathering statistics for each of said plurality of scan chain faults based on the diagnostic fault simulation, wherein the statistics for each scan chain fault includes failures predicted by the scan chain fault; modeling at least two defects at different locations on the scan chain by compositing the statistics for a subset of the scan chain faults for each of said defects; and determining the location of a defect on the scan chain based on which one of the composites is closest to predicting failure measurements at a tester.