The U.S. Air Force, facing a shortage in some ranks and job categories, has allowed enlisted troops who have not qualified for promotion to stay in their current rank for another two years. This means that E-4s (Senior Airman) may stay for 12 years, E-6s (Tech Sergeant, and so on) for 24 years, E-7s for 26 years and E-8s for 28 years. The Army and Navy have also used this technique to solve shortages. The American military has, since World War II, practiced an "up or out" personnel policy for officers and enlisted troops. If you don't qualify for promotion within a certain number of years, you have to leave. This is quite an incentive, as you don't qualify for retirement benefits until you are in for at least twenty years. At that point you retire with half your active duty pay. If you stay for at least thirty years, you retire with 75 percent of your active duty pay. Often, an NCO is stuck in rank because a particular job category has too many people in the next highest rank. So only the most capable get a chance for promotion. Rather than lose experienced and capable people, the time to get promoted is extended two years. These extensions are issued from time to time rather than lose experienced people who will be needed after the promotion glut goes away because of retirements.