Federal economic stimulus checks started being mailed Friday, a week ahead of the posted schedule, the Internal Revenue Service (IRS) has announced.

Some, but not all, taxpayers will receive their checks early. There is no way to tell who will or will not get their check early, the IRS said in a news release.

According to the release, the top questions about Economic Stimulus Payments (ESPs) the IRS continues to get from taxpayers are:

* Why am I getting a paper check when I chose direct deposit on my 2007 tax return?

"If you requested that your electronic filing or tax preparation fee be deducted from the amount of your refund, you have entered into a financial agreement with the tax preparation provider or e-file software company for a Refund Anticipation Check (RAC)," the IRS said.

"Both Refund Anticipation Checks and Refund Anticipation Loans (RALs) are bank products. The money that is deposited into your bank account comes from the financial institution associated with your electronic filing software provider or tax preparation provider, not directly from the IRS.

"Taxpayers who use RALs, RACs or enter into similar loans or financial agreements with their tax professional or tax software companies will not receive their economic stimulus payments by direct deposit.

"Instead, they will get a paper check that will be issued according to the schedule that has been published for paper check distribution."

* I received my stimulus payment, but it was less than I expected. Why?

"Your economic stimulus payment is based on information provided on your 2007 income tax return," the news release said.

"Many, but not all, taxpayers qualify for the maximum basic payment of $600 for singles or $1,200 for married couples. Many parents are also receiving an additional $300 for each qualifying child, born after Dec. 31, 1990."

However, individual payments may be less than the maximum for one or more of the following reasons:

* You are single and your net income tax liability is less than $600. If you file Form 1040, your net income tax liability is the amount shown on Line 57, plus the amount on Line 52.

* You are married, and your net income tax liability is less than $1,200.

* You are single, and your adjusted gross income (AGI) is more than $75,000. On Form 1040, AGI is the amount on Line 37.

* You are married filing a joint return, and your AGI is more than $150,000.

* You owe back taxes that reduced your payment.

* You have non-tax federal debts such as unpaid student loans or child-support obligations that reduced your payment.

"Around the time you receive your payment, you will also receive a notice from the IRS explaining how your payment was calculated," the IRS news release said.

"It is important to keep this notice as a record of your economic stimulus payment. In addition, you'll get a separate notice if you owe back taxes or non-tax debts that were offset or deducted from your stimulus payment."