I threw out zero because it seems like a non-response to me in a lot of questions. Someone giving a number shows they have at least responded with a non-default and did donate something, while someone leaving a zero may be simply equivalent to people who left empty responses.

(While I'm commenting on my analysis: from a utilitarian perspective, comparing logs doesn't even make sense; personal utility may follow some sort of logarithm in money, but charities don't - the problems are just too big for any one person.)