Private colleges increased their tuition by an average of 3.9 percent in 2012-13, the smallest rise in four decades, the National Association of Independent Colleges and Universities announced Thursday. The association said its survey, which included responses from 445 of its 960 members, also found that the average institution's financial aid budget rose by 6.2 percent. The data come at a time of heightened pressure from politicians and the public for colleges to keep their charges within reach of students and families.