Article: The Paris region hosts France's highest concentration of the grandes écoles – 55 specialised centres of higher-education outside the public university structure. The prestigious public universities are usually considered grands établissements. Most of the grandes écoles were relocated to the suburbs of Paris in the 1960s and 1970s, in new campuses much larger than the old campuses within the crowded city of Paris, though the École Normale Supérieure has remained on rue d'Ulm in the 5th arrondissement. There are a high number of engineering schools, led by the Paris Institute of Technology which comprises several colleges such as École Polytechnique, École des Mines, AgroParisTech, Télécom Paris, Arts et Métiers, and École des Ponts et Chaussées. There are also many business schools, including HEC, INSEAD, ESSEC, and ESCP Europe. The administrative school such as ENA has been relocated to Strasbourg, the political science school Sciences-Po is still located in Paris' 7th arrondissement and the most prestigious university of economics and finance, Paris-Dauphine, is located in Paris' 16th. The Parisian school of journalism CELSA department of the Paris-Sorbonne University is located in Neuilly-sur-Seine. Paris is also home to several of France's most famous high-schools such as Lycée Louis-le-Grand, Lycée Henri-IV, Lycée Janson de Sailly and Lycée Condorcet. The National Institute of Sport and Physical Education, located in the 12th arrondissement, is both a physical education institute and high-level training centre for elite athletes.

Now answer this question: During which time frame were most grandes ecoles relocated?
1960s and 1970s