 While brevity usually defines an essay, voluminous works like John Locke's "An Essay Concerning Human Understanding" and Thomas Malthus's "An Essay on the Principle of Population" are counterexamples.In some countries (e.g., the United States and Canada), essays have become a major part of formal education.Secondary students are taught structured essay formats to improve their writing skills; admission essays are often used by universities in selecting applicants, and in the humanities and social sciences essays are often used as a way of assessing the performance of students during final exams.