A partial scan selection strategy is proposed in which flip-flops are selected via newly proposed dynamic reachability and observability measures such that the remaining hard-to-detect faults are easily detected. This is done by taking advantage of the information available when a target fault is aborted by the test generator. A partial scan selection tool, IDROPS, has been developed which selects the best and smallest set of flip-flops to scan that will result in a high fault coverage. Results indicate that high fault coverage in hard-to-test circuits can be achieved using fewer scan flip-flops than in previous methods.