While the American military has increased the professional training for its troops, this has been accompanied by an increase in the number of enlisted personnel who have college degrees. In the air force, 4.4 percent of enlisted troops have four year degrees, while 14.4 percent have two year degrees. These degrees tend to be concentrated among the senior NCOs. For example, in the navy, 56 percent of those in the top enlisted ranks (E-9s, the Master Chief Petty Officers) have college degrees. But there are some very highly educated among the lower ranking NCOs. There are 17 Petty Officers Third Class (E-4), with PhDs. Basically, the educational attainment of enlisted personnel has gone up with the general population over the last twenty years. In addition, the military only enlists those in the top 75 percent of the population (in terms of education.) The military has long paid for troops to take college courses in their spare time, and provides instructors in remote locations. Warfare is getting more complex, especially in terms of the equipment and techniques. Better educated troops are needed to make it all work.