Greg Mankiw explains one of his:

Similarly, when I write a column for the New York Times, I am not good at predicting how much it will get people talking. As a result, I monitor the subsequent blogosphere commentary to judge if the article is a snore (like most things that get published) or if it is commanding attention. At the very least, I expect my articles to be noteworthy enough that within a few days Brad DeLong will call me a moronic hypocrite. I hope my articles incite some wider commentary as well, but I never know in advance.

Personally, I don't despair if it takes Brad more than a few days to unleash the vitriol.  I know that eventually, he'll get around to it.