1. A method for determining a subset of a set of library cells of a library cell type for a timing arc of the library cell type, the method comprising: receiving a first generic logical effort value for the timing arc of the library cell type, wherein the first generic logical effort value is determined by aggregating specific logical effort values across multiple library cells of the library cell type receiving a generic parasitic delay value for the timing arc of the library cell type; and determining, by using a computer, the subset of the set of library cells of the library cell type, wherein said determining comprises: excluding library cells in the set of library cells of the library cell type whose specific logical effort values for the timing arc are greater than the first generic logical effort value by more than a first threshold, and excluding library cells in the set of library cells of the library cell type whose specific parasitic delay values for the timing arc are greater than the generic parasitic delay value by more than a second threshold.