United States Air Force
USAF rank is divided between enlisted airmen, non-commissioned officers, and commissioned officers, and ranges from the enlisted Airman Basic (E-1) to the commissioned officer rank of General (O-10). Enlisted promotions are granted based on a combination of test scores, years of experience, and selection board approval while officer promotions are based on time-in-grade and a promotion selection board. Promotions among enlisted personnel and non-commissioned officers are generally designated by increasing numbers of insignia chevrons. Commissioned officer rank is designated by bars, oak leaves, a silver eagle, and anywhere from one to four stars (one to five stars in war-time).[citation needed]
How are Enlisted promotions in the USAF decided? 
based on a combination of test scores, years of experience, and selection board approval
Enlisted promotions are granted based on a combination of test scores, years of experience, and selection board approval while officer promotions are based on time-in-grade and a promotion selection board.
Enlisted promotions