In the pre-independence period, education in the country followed the Portuguese system. Education under the independent government has been patterned after the program of popular education carried out in the liberated areas of Guinea-Bissau. The program stresses universal literacy and primary skills, with advanced education geared toward agricultural and technical skills for production. In 1998, primary schools had 91,177 students and 3,219 teachers, with a student to teacher ratio of 29 to 1. Secondary schools had 31,602 students and 1,372 teachers in the same year. As of 1999, 99% of primary-school-age children were enrolled in school, while 48% of those eligible attended secondary school. Primary education is compulsory and lasts for six years. Projected adult illiteracy rates for the year 2000 stand at 26.5% (males, 15.7%; females, 34.7%). As of 1999, public expenditure on education was estimated at 4.4% of GDP.