Menzie Chinn takes on stimulus and forecasting: Part 2

I linked favorably to a post by Menzie about the stimulus a little while ago and said:

I'm not sure if I agree with the last parenthetical sentence. If GDP is a percentage point lower than what was expected conditioning on the expected passage of the stimulus, why does that mean that "individuals who ascribe the worse-than-expected performance of the economy to the stimulus package cannot look to the ex post GDP realizations for support for their arguments"? In fact, I don't see how it supports either a pro or anti-stimulus argument. GDP could be worse than expected because the economy was worse than the consensus, the stimulus did not live up to the Obama administration's expectations, or the very, very small chance the stimulus had a negative effect (none of these are mutually exclusive). Menzie makes a good point when he states that very little of the stimulus was spent in Q1, and thus should have little effect on Q1 GDP. I can see how that means it's not right to look at Q1 numbers and conclude the stimulus was ineffective, but not the logical conclusion Menzie makes.

Menzie replied via the comment board on his blog and said this:

As of early Feb., one has an information set incorporating passage of the stimulus bill, if not with 100% certainty, then pretty high. Then you have the actual realization of GDP. You can decompose the "surprise" into a part that it "new" information regarding the state of the economy, and "new" information regarding stimulus composition and effectiveness. I set the new information regarding the latter at near zero. You could try your own decomposition; I'd welcome finding out your conclusions.

Thanks to Menzie for the reply. I understood the major point of his augment, but got a little confused with his conclusion. I said, "... he states that very little of the stimulus was spent in Q1, and thus should have little effect on Q1 GDP. I can see how that means it's not right to look at Q1 numbers and conclude the stimulus was ineffective..." From what Menzie said in his reply, I take it that's about what he meant (ie that GDP was lower that what was anticipated was due to errors in forecasting, and not an ineffective stimulus). Sorry Menzie, I guess I misunderstood what you wrote.