[Elephant Walk]

"Nevertheless, even good guesswork is still guesswork. As the George Mason economist Arnold Kling says, “it is literally an impossible task” to accurately make the sort of projections the CBO specializes in. “We don’t do controlled experiments in economics,” Kling says. “So when you’re talking about figuring out the effects of health care policy, it’s very difficult.”

Part of the difficulty is that the CBO is trying to replicate systems it can’t really see. To understand the problems with building an economic model, consider what it takes to make a working scale-model train. To build that train, you’d first need accurate information about how the full-size train works: how big its parts are, at what speed those parts move, its power consumption and control system. Imagine trying to build a model train without ever being allowed to look inside the engine compartment. A smart engineer would be able to make reasonably educated guesses about the internal workings by measuring the outside and by looking at various external controls, but those guesses would almost certainly come with a high margin of error.

That’s no small part of the problem for the CBO. When scoring legislation, they’re essentially trying to build small-scale working models of systems using fairly limited data sets. For example, according to Reischauer and Billheimer, the National Health Interview Survey provided CBO analysts with data on “health insurance coverage, health states, use of health services and socioeconomic variables.” But these sources provided “no data on premiums or cost-sharing requirements, and no indication of the exact share of premiums paid by employers.”

http://reason.com/archives/2009/12/08/the-gatekeeper/1

I don't personally like the corporatist bill that was passed. But I doubt the Republican plan is much better.