Tau-b or Not Tau-b: Measuring Alliance Portfolio Similarity
Ritter, Jeffery M.
Signorino, Curtis S.
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 similiarity, S, which avoids many of
the problems associated with tau-b, and we use data on
alliances among European states to compare 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 alliances,
trade, UN votes, diplomatic missions, and other types of state
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