DMI assigns letter grades to Members of Congress each year based on their votes for or against the middle class. For votes taken in 2007 and 2008, the following formula is used to determine letter grades:

A letter grade of ‘A+’ is awarded for a perfect record of voting for the middle class. A letter grade of 'A' is awarded to legislators who voted the middle-class position on 90% or more of the votes they cast. A letter grade of ‘B’ is awarded for an 80% voting record or better; ‘C’ for a 50% record or better; ‘D’ for a 40% record or better; and ‘F’ for legislators who voted for the middle class less than 40% of the time.

Missed votes – whether due to absence or abstention – do not count in the percentage of votes used to calculate the grade. But a Member of Congress who has missed five or more votes in a year receives a grade of incomplete (INC.) A legislator who was not in office during a given year receives a grade of N/A.

In 2004 and 2005, the grading formula did not include 'A+' grades. Legislators with a perfect voting record received 'A's. In these years, in which fewer votes were considered overall, legislators received a grade of incomplete for missing three or more votes.

In 2003, a slightly different formula was used for the legislator grades. A letter grade of ‘A’ was awarded to legislators who voted the middle-class position on 100% of the votes they cast. A letter grade of ‘B’ was awarded for an 80% voting record or better; ‘C’ for a 60% record or better; ‘D’ for a 50% record or better; and ‘F’ for legislators who voted for the middle class less than 50% of the time. The same considerations regarding missed votes apply as in the later years.

In all cases, grades for the House and Senate on a particular bill or for the year as a whole are determined using the formula introduced in 2004. If 90% or more of Senators or Representatives vote for the middle-class, their chamber gets an A and so on. Those who do not vote are not counted in the number of legislators used to calculate the grade.