It's the same as why you wouldn't measure your weight in ounces, but neither would you weigh something very light in pounds - you want a convenient number which is easy to interpret. PC is short for parsec and it's a unit of measurement equal to 3,000,000,000,000,000,000 cm!
Take your annual covered earnings (the earnings on which you pay Social Security payroll taxes), multiply it by that year's index factor and you will obtain your indexed wage for each year you have worked. Next, add up all these indexed wages and divide them by 35 to determine your average wage.
Box 3 (Social Security Wages) represents the portion taxable for social security purposes. Social security wages are calculated as Federal Taxable Gross (Box 1) plus Retirement Deductions (Box 12). The maximum social security wage amount for 2015 ande 2016 is $118,500. Box 5 (Medicare Wages) represents the portion taxable for Medicare tax purposes. Unlike Social Security wages, there is no cap for Medicare wages. One of the most common differences between box 1 and boxes 3 and 5 is the amount of employee contributions to deferred compensation plans such as traditional 401(k) and 457.