As it has in the past several years, the budget plan defends federal pay levels against studies that compare the average federal salary with the overall national average, by stressing the differing nature of federal work. It says that 56 percent of federal employees work in one of the nine highest-paying occupational groups, compared with 36 percent in the private sector, while only 4 percent are employed in the three lowest-paid occupations, compared with 12 percent. Even in large firms, it says, the percentage of workers with a masters degree or higher is less than half of that of the federal sector, and federal workers overall are more experienced, older and more concentrated in higher-cost metropolitan areas–all correlated to higher pay. It adds that even with a 1.6 percent raise in 2017, federal salary rates will have fallen 9 percent behind the growth of private sector pay since 2009 as measured by the Employment Cost Index (to which the across the board component of federal raises are supposed to be linked, with locality pay to be paid in addition). Other factors also have eroded federal compensation, it says, including the increases in contributions toward retirement required from those first hired after 2012, and tight budgets that have limited the use of payments such as student loan reimbursements.