Very convenient.
Everyone, and I mean damn near everyone, has more withdrawn from their pay than the government keeps at the end of the year. It is why you hear everyone bitching about giving the government an interest free loan.
Then there is the earned income tax credit, and the standard deduction, or itemized deductions. All of this is calculated by you or a tax return specialist (H&R Block, or a CPA or whomever) and sent to the IRS and they send you money back.
You look at how much they send you back, compare that against your total withholding from the last pay stub of the year, take some common sense and a calculator, and arrive at an effective tax rate.
You have one piece of the equation there.
In no way is that your tax rate.
If you believe that to be your tax rate, you have lost forever any right to call anyone stupid.