What percentage of doubleheaders are swept?

This question originated from the (dis)advantage that the REd Sox had by having their Monday night game rained out, while the Yankees played. I heard this stat for the first time yesterday and was suprised by it. The stat bureau indicated that a majority of baseball insiders were shocked by it as well. Correct answer to be posted later.

FYI: Stat reported on doubleheaders since 1990's through present.