What is a "fair share"?

I am always hearing this from democrats, usually in the context that the rich should pay more. However, I have never heard what a “fair share” is. So, I ask that question now; given the facts I will lay out below, that is a fair share of the income tax burden for each class or income group?

Here is what I used. I started with the 2009 Statistics of Income (SOI) produced by the Internal Revenue Service. You can find them at irs.gov.

Adjusted Gross Income (AGI) – income before deductions, exemptions, and credits.

Tax Liability (TL) – the amount of tax actually paid (ie – the check you write to the government)

Effective tax rate (ETR) – TL / AGI

Lower class – AGI below $75k

Lower Middle class – AGI between $75k and $200k

Upper Middle class – AGI between $200k and $1m

Upper class – AGI over $1m

ETR for each class

Lower – 4.84%

Lower Middle – 10.49%

Upper Middle – 20.75%

Upper – 24.42%

Percent of total taxes collected that are paid by each class

Lower – 16.04%

Lower Middle – 33.81%

Upper Middle – 29.75%

Upper – 20.50%

Percent of total population for each class based on returns filed

Lower – 79.42%

Lower Middle – 17.78%

Upper Middle – 2.62%

Upper – 0.17%

Your answer should include the class breakdown (feel free to used other breakdowns, this is just my opinion), ETR, and your reasoning for your class and ETR breakdown.