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For self-employed incomes, the Court has discretion. The Judge may either use a mean (average), or a median (middle number) or mode (whatever occurs most often) depending on your income, the regularity with which you get paid, etc.
It is absolutely fair to ask that they average three years of income. However, the ex does not have to agree. If they do not agree, the Judge makes a decision based on what they believe is the fairest representation of your income over the year(s).
So there IS some subjectivity there.
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