if education is seen as a luxury good, it may be that richer households are seeking out educational attainment as a symbol of status, rather than the relationship of education leading to wealth. Educational advance is not the only variable for economic growth, though, as it only explains about 14% of the average annual increase in labor productivity over the period 1915-2005. From lack of a more significant correlation between formal educational achievement and productivity growth, some economists see reason to believe that in today's world many skills and capabilities come by way of learning outside of traditional education, or outside of schooling altogether. An alternative model of the demand for education, commonly referred to as screening, is based on the economic theory of signalling. The central idea is that the successful completion of education is a signal of ability. Marxist critique Although Marx and Engels did not write widely about the social functions of education, their concepts and methods are theorized and criticized by the influence of Marx as education being used in reproduction of capitalist societies. Marx and Engels approached scholarship as "revolutionary scholarship" where education should serve as a propaganda for the struggle of the working class. The classical Marxian paradigm sees education as serving the interest of capital and is seeking alternative modes of education that would prepare students and citizens for more progressive socialist mode of social organizations. Marx and Engels understood education and free time as essential to developing free individuals and creating many-sided human beings, thus for them education should become a more essential part of the life of people unlike capitalist society which is organized mainly around work and the production of commodities. Financing and provision In most countries school education is predominantly financed and provided by governments. Public funding and provision also plays a major role in higher education. Although there is wide agreement on the principle that education, at least at school level, should be financed mainly by governments, there is considerable debate over the desirable extent of public provision of education. Supporters of public education argue that universal public provision promotes equality of opportunity and social cohesion. Opponents of public provision advocate alternatives such as vouchers. Inequalities in education financing Since the 1960s, government expenditure on education for low and middle-income countries generally increased while spending on education for high-income countries remained relatively constant. Based on educational funding in