Sentence:  In 2000, David X. Li, a banker with a doctorate in statistics who was then at RiskMetrics, part of J. P. Morgan Chase, began using mathematical functions called Gaussian <WSD>copulas</WSD> to estimate the likelihood of corporations' dying in unison. 
Meaning of the word marked with <WSD> tags:  A function that represents the association between two or more variables, independent of the individual marginal distributions of the variables.