Tau-b or Not Tau-b: Measuring the Similarity of Foreign Policy Positions

The pattern of alliances among states is commonly assumed to reflect the extent to which states have common or conflicting security interests. For the past twenty years, Kendall's τb has been used to measure the similarity of nations' “portfolios” of alliance commitments. Widely employed indicators of systemic polarity, state utility, and state risk propensity all rely on τb. We demonstrate that τb is inappropriate for measuring the similarity of states' alliance policies. We develop an alternative measure of policy portfolio similarity, S, which avoids many of the problems associated with τb, and we use data on alliances among European states to compare S to τb. Finally, we identify several problems with inferring state interests from alliances alone, and we provide a method to overcome those problems using S in combination with data on alliances, trade, UN votes, diplomatic missions, and other types of state interaction. We demonstrate this by comparing the calculated similarity of foreign policy positions based solely on alliance data to that based on alliance data supplemented with UN voting data.