The right to education has been recognised as a human right in a number of international conventions, including the International Covenant on Economic, Social and Cultural Rights which recognises a right to free, compulsory primary education for all, an obligation to develop secondary education accessible to all with the progressive introduction of free secondary education, as well as an obligation to develop equitable access to higher education, ideally by the progressive introduction of free higher education. In 2021, 171 states were parties to the Covenant. In 2019, an estimated 260 million children worldwide did not have access to school education, and social inequality was a major cause. The Human Rights Measurement Initiative measures the right to education for countries around the world, based on their level of income.