Methodology: How The Rankings Were Reckoned

Generosity can be difficult to pin down. Many of America's most open-handed philanthropists work hard to keep their gifts quiet. Public information is rarely comprehensive. And stocks can be volatile, causing large pledges of shares to shift in value over time. So to compile BusinessWeek's annual ranking, we relied heavily on news reports, foundation filings, and interviews. Resources included The Chronicle of Philanthropy, the Forbes 400 Richest Americans, Prospect Information Network, and nonprofit GuideStar's online database of IRS foundation filings. Using these data, we ranked the 50 Most Generous Philanthropists by what they've pledged and given in the past five years. We also estimated their total lifetime contributions and presented these figures as a percentage of each candidate's current net worth.

Pledges counted, too. The reason: We believe donors who vow to give their fortunes away in the future deserve to be lauded, not only for their generosity but also because they often inspire others to greater charitable endeavors. Still, valuing such pledges can be tricky. In many cases, the actual gift amount changes after the pledge is made. And sometimes philanthropists struggle to make the payments. For example, since William and Claudia Coleman made their $250 million pledge to the University of Colorado in 2001, their stock in the company he co-founded, BEA Systems Inc. (BEAS ), has dropped in value by 87%. The Colemans are expected to fulfill their pledge on an extended time line. For consistency, we counted all pledges at their value at the time they were initially announced. We discounted only those that had been formally revoked.

We may have missed some big gifts. But our ranking captures the collective generosity of givers who have risen to the challenge Andrew Carnegie issued to the well-heeled in his 1889 essay, The Gospel of Wealth. He wrote that the goal of the rich should be to return "their surplus wealth to the mass of their fellows in the form best calculated to do them lasting good." BusinessWeek's Top 50 would make Carnegie proud.

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