The analytic hierarchy process (AHP) provides a decision-analysis framework to model unstructured problems in almost every kind of discipline, whether social science, aerospace engineering, or nuclear reactor safety analysis. As common-cause failure (CCF) has been a major element of incidents and accidents in terrestrial nuclear power reactors because of high redundancy built into the systems and susceptibility of these redundant systems to CCF mechanisms, ad hoc approaches used to be taken to address vulnerabilities to CCF by designers or operating staff of the plants. We show in this paper how the AHP in conjunction with goal-tree success-tree (GTST) methodology can be used to identify an optimal CCF-defense strategy under various constraints (e.g., the largest safety impact, the smallest cost, and the least operator burden). This work demonstrates applicability and effectiveness of the AHP decision-analysis technique in CCF-defense assessment with a novel introduction of the GTST methodology as a tool to construct a hierarchical decision tree for the AHP. The combined approach based on AHP and GTST methodologies can be used not only for CCF-defense assessment but also for any other multicriteria decision analysis requiring priority setting.