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The switching activity of scan-based tests for delay faults is considered as a test compaction heuristic. Two test compaction processes based on the switching activity are described. The results of several experiments are presented where test sets consisting of tests with different switching activity are compared based on their size as well as coverage of untargeted faults. The results demonstrate that test sets where the tests have higher switching activity are smaller. Their untargeted fault coverage is comparable, and sometimes even higher, than that of larger test sets for the same target faults. To avoid overtesting due to high switching activity it is possible to consider functional broadside tests. For other types of tests it is possible to bound the switching activity such that it would not exceed that possible during functional operation.