In 2014 and 2016 I wassubject to and used Alternative Min Tax . However, 2015 doe not have the AMT calculation even though taxable income was high

[MiriamF]

Whether you are subject to AMT depends not upon the size of your income, but on the source of your income and the deductions you are claiming. The purpose of the AMT is to close loopholes that some wealthy taxpayers use to avoid paying tax.

This article explains more about what triggers the Alternative Minimum Tax.