Embrechts, McNeil and Straumann [1999,2000] clarified the essential concepts of dependence and correlation that allowed the introduction to finance of a mathematical concept called the Wikipedia.) ">copula function. Technically, a copula is a function that links univariate margins to the full multivariate distribution resulting in a joint distribution function of N standard uniform random variables. This is a powerful concept in the aggregation of individual risks. (More information about this statstical Wikipedia.) ">Copula Function can be found on Wikipedia.)