Tau-b or Not Tau-b: Measuring the Similarity of Alliances and Interests

Date Published:

Abstract:

The pattern of alliance commitments 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 tau–b has been used to measure the similarity between two nations' "portfolios" of alliance commitments. Widely employed indicators of systemic polarity, state utility, and state risk propensity all rely upon tau–b. We demonstrate that tau–b is inappropriate for measuring the similarity of states' alliance commitments. We develop an alternative measure of alliance portfolio similarity, S which avoids many of the problems associated with tau– b, and we use data on alliance among European states to comparare the effects of S versus tau–b in measures of utility and risk propensity. Finally, we identify several problems with inferring state interest from alliance commitments and we provide a method to overcome those problems using S in combination with data on alliance, trade , UN votes, diplomatic missions, and other types of state interaction. We demonstrate this by comparing the calculated "similarity of interests" when using solely alliance data versus that using alliance data supplemented with UN voting data.

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Notes:

Working Paper 97–07, Weatherhead Center for International Affairs, Harvard University, 1997.