When economists speak about education and human capital, they usually mean formal education. It is provided in schools and universities by formally qualified teachers. These are imparting knowledge that is laid down in curricula, and the result of the learning process is testified by certificates and diplomas conferred to those students who passed exams. Hence economists usually measure the availability of human capital in a society by the average number of years citizens attended schools and universities. Yet is formal education the only source of human...