Is it possible to assess how charitable someone is, apart from the size of their donations? Last year Warren Buffett made the biggest donation in history but, relatively speaking, is he more or less generous than a middle-class American who volunteers in a primary school?

Over the years, studies have ranked giving in the US and, in the process, sometimes rankled states that landed on the stingy side. They have also stimulated a broader discussion over whether generosity can - or should - be measured.