It provides, with the linked graphic, an explanation of methodology.
"The Economist’s first-ever college rankings are based on a simple, if debatable, premise: the economic value of a university is equal to the gap between how much money its graduates earn, and how much they might have made had they studied elsewhere. Thanks to the scorecard, the first number is easily accessible. The second, however, can only be estimated. To calculate this figure, we ran the scorecard’s earnings data through a multiple regression analysis, a common method of measuring the relationships between variables."