We used the Tax Policy Center (TPC) microsimulation model to estimate tax expenditures and distributional effects. The model calculates tax liabilities for a weighted sample of individual income tax returns and can be used to analyze current law as well as alternative policies that change the tax code, including tax rates, the AMT, exemptions, deductions, and credits. It is based on the 2001 Public Use File (PUF) produced by the Statistics of Income Division (SOI) of the IRS. The PUF includes sample weights that can be used to produce population estimates. The PUF and sample weights are extrapolated to later years based on more recent SOI published data and Congressional Budget Office (CBO) economic projections, and imputations from other data sources of items not reported on tax returns. The model assumes that, conditional on income, deductions, family status, etc., individuals legally minimize their tax owed (for example, by correctly deciding whether to itemize deductions), but it does not incorporate any other types of behavioral responses, such as changes in labor supply, investment or consumption patterns in response to tax incentives.

It is our belief that journals should publish the results of replication attempts-favorable or unfavorable.''-Dewald, Thursby, and Anderson (1988) ''Econometric software has bugs.''-McCullough and Vinod (1999) ''. .. [R]eplicable economic research is the exception and not the rule.''-Anderson et al. (2005) With this issue, Public Finance Review issues an open call for papers that report the results of attempts to replicate significant empirical research in public economics, published in this journal or elsewhere. The Scientific Need for Replication Studies A basic requirement for scientific integrity is the ability to replicate the results of research, and yet, with some occasional historical exceptions,

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This paper incorporates retirement saving incentives into the Tax Policy Center microsimulation model and analyzes the distributional effects of current tax preferences for saving. As a share of income, tax-preferred saving incentives provide the largest benefits to households with income between $75,000 and $500,000, roughly the 80 th to 99 th percentile of the income distribution. In 2004, the top 20 percent of tax filing units by income will receive 70 percent of the tax benefits from new contributions to defined contribution plans and almost 60 percent of IRA tax benefits.

We examine retirement savers' choices between frontand back-loaded tax incentives, such as traditional and Roth IRAs. With equal dollar contribution limits, back-loaded plans shelter more funds than front-loaded plans. This implies that Roth IRAs can be the preferred choice even for investors who expect their tax rates to fall in retirement. Empirically, we examine how marginal tax rates have varied between 1982 and 1995 for a sample of taxpayers and calculate both ex ante and ex post effective tax rates on front-loaded IRAs. The average effective tax rate on traditional IRA contributions made in 1982 and withdrawn in 1995 was-30 percent. Changes in tax law after 1982 reduced tax rates considerably. Holding tax law constant, the average effective tax rate on IRAs was about-11 percent. These results occur because the tax rate in retirement is lower for most people than the rate while working. In contrast, the effective tax rate on Roth IRAs is always zero. Despite the lower average effective tax rate on traditional IRAs, many taxpayers in the sample would have benefited from contributing to a Roth IRA instead of a traditional IRA, due to the difference in effective contribution limits.