But here we run into another counterfactual problem; we cannot separate the impact of fiscal policy from that of monetary.
Unlike the Connecticut and Iowa evaluations, they were not set up to separate the impact of welfare policy from the strong economy and other factors.
The analysis did not try to separate the impact of more productivity -- the extra 40 minutes a day -- from other factors, like a reduction in class size to 20 students from as many as 30, teacher training and new curriculums.
Yet Mr. Ryan, a Wisconsin Republican, went on to separate the impact of Mr. Obama's words on long-term budget negotiation from their impact on the short-term challenge of persuading Congress to raise the federal debt limit.
Other senior intelligence and counterterrorism officials offer a dissenting view, saying that it is difficult, if not impossible, to separate the impact of the messages between the Qaeda leaders from Mr. Snowden's overall disclosures, and that the decline is more likely a combination of the two.
Though Mr. Koomey was unable to separate the impact of the recession from that of energy-saving technologies, the decline in use is surprising because data centers, buildings that house racks and racks of computers, have become so central to modern life.
Esteban Buch argues that you cannot separate the impact of one from the other.
Gibson agrees that it is difficult to separate the impact of forest fragment size from that of the rats.
