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Another expert here:
If you are being paid as an independent contractor (you will receive a 1099-MISC instead of a W-2) for the locums work, then you will be reporting the income as a sole proprietor and can establish a retirement plan for that business income. Assuming you don't already contribute to a 401(k) through your salary as a resident, then you could establish a solo 401(k) and contribute $17,000 as an elective deferral and another approximately 20% of your net profit on Schedule C as an employer contribution. However, in order to do this the plan document would have to be executed and established by 12/31/12. The entire funding of the plan can be up until your tax return filing deadline plus extension.
If you have already contributed the maximum $17,000 to a 401(k) plan through your resident employer, then you could establish a SEP IRA instead and contribute approximately 20% of your net profit on Schedule C. This plan does not have to be established by 12/31/12. It can be established and funded up until your tax return due date plus extension.
There are 2 types of funding in a 401(k). One is considered an employee elective deferral (up to $17,000 if you are under age 50) and the other is considered an employer contribution. If you are a sole proprietor, then you are considered both an employee and the employer of the sole proprietorship. Hence, you can contribute $17,000 as an employee elective deferral and approximately $11,500 - $12,000 as an employer contribution (based on $60,000 of net profit). So if you contribute the maximum amount, your federal taxable income will drop by about $29,000 saving you approximately $7,250 in federal income taxes. Even though your federal income tax would be decreased by this amount, you will still owe self-employment social security taxes on the $60,000 of locum income so this will be about $6,800.
The 40% tax rate is a rough estimate when you add federal, state, and self-employment social security taxes.