At our annual dinner this year, economist Rick Evans spoke about open source tax policy models. (We excerpted his talk for an episode of Policy Today.) He noted that there are three groups at the federal level who estimate the effects of tax policy changes: the Joint Committee on Taxation, the Tax Policy Center, and the Tax Foundation. But the three groups each have their own proprietary models, so it’s next to impossible for outsiders to assess the different approaches they take and discern which model is most likely to provide the best estimates.

This led to Evans’ work on an open source model, which would allow anyone to tinker with the assumptions and parameters in the model to estimate the impacts of a policy change.

Today provides an example of why that would be helpful: Senator Rand Paul, who is running for president, has proposed repealing “the entire IRS tax code—more than 70,000 pages—and replace it with a low, broad-based tax of 14.5% on individuals and businesses.”

The Tax Foundation analyzed his proposal and found it would “grow the economy by 9.4 percent in the long run, create 1.4 million jobs, and cost $2.97 trillion over ten years on a static basis and $960 billion when accounting for economic growth.”

Howard Gleckman at the Tax Policy Center pushes back:

Paul is probably underestimating the cost of his plan. He claims he’d cut taxes (and thus increase the debt) by $2 trillion over 10 years. But he’s using a very aggressive analysis that assumes powerful growth effects from the tax reductions. . . .