technological progress in accounting for sustained increases in output per worker. His 1983 dissertation, supervised by José Scheinkman and Robert Lucas Jr., amounted to constructing mathematical representations of economies in which technological change is the result of the intentional actions of people, such as research and development. It led to two Journal of Political Economy articles published in 1986 and 1990, respectively, which started endogenous growth theory. He taught at the University of Rochester, the University of Chicago, the University of California, Berkeley, Stanford University and New York University. At New York University, he founded the Marron Institute of Urban Management. Its objective is to assist cities in planning its future developments, specifically improving the safety, health, and mobility of citizens. Business Romer temporarily left academia in 2001 to found Aplia, a company which produces online problem sets for college students. Students have submitted upwards of 2.4 billion answers to homework problems on the Aplia website. Aplia was purchased in 2007 by Cengage Learning. He is credited with the quote "A crisis is a terrible thing to waste," which he said during a November 2004 venture-capitalist meeting in California. Although he was referring to the rapidly rising education levels in other countries compared to the United States, the quote became a rallying concept for economists and consultants looking for constructive opportunities amid the Great Recession. Charter cities Romer has attempted to replicate the success of charter cities and make them an engine of economic growth in developing countries. He promoted this idea in a TED talk in 2009, and he has argued that with better rules and institutions less developed nations can be set on a different and better trajectory for growth. In his model, a host country would turn responsibility for a charter city over to a more developed trustee nation, which would allow for new rules of governance to emerge. People could "vote with their feet" for or against these rules. The government of Honduras considered creating charter cities, though without the oversight of a third-party government, which some argue is neo-colonialism. Romer served as chair of a "transparency committee" but resigned in September 2012 when the Honduran government agency responsible for the project signed agreements with international developers without involvement of the committee. World Bank He became World Bank Chief Economist in October 2016. He resigned on 24 January 2018, following a controversy in which he stated