The Croatian language is Croatias official language, but the languages of constitutionally-recognised minorities are officially used in some local government units. Croatian is the native language identified by 96% of the population. A 2009 survey revealed that 78% of Croatians claim knowledge of at least one foreign language—most often English language. The largest religions of Croatia are Roman Catholic Church (86.3%), Eastern Orthodox Church (4.4%) and Islam (1.5%). Literacy in Croatia stands at 98.1%. The proportion of the population aged 15 and over attaining academic degrees has grown rapidly since 2001, doubling and reaching 16.7% by 2008. An estimated 4.5% of GDP is spent for education. Primary and secondary education are available in Croatian and in the languages of recognised minorities. Croatia has a universal health care system and in 2010, the nation spent 6.9% of its GDP on healthcare. The net monthly income in September 2011 averaged 5,397 Croatian kuna ( ). The most significant sources of employment in 2008 were wholesale and retail trade, the manufacturing industry and construction. In October 2011, the unemployment rate was 17.4%. Croatias median equivalent household income tops the average Purchasing Power Standard of 2004 enlargement of the European Union, while trailing the EU average. The 2011 census recorded a total of 1.5 million private households; most owned their own housing.. How many years did it take the percentage of Croatians earning academic degrees to double?