While each country is free to choose its team by whatever means it seems appropriate, the selection process usually involves holding regional and national olympiad competitions. Many countries hold "training camps" for its top students, where mentors from the country give the students accelerated college-level courses in chemistry with an emphasis on the topics covered in that year's preparatory problems as well as practical training. It is agreed that such training programs must not exceed a total duration of two weeks but there are allegations every year that some countries exceed this limit by months or even years. Another concern is that some countries tend to bring the same students to the competition year after year, which helps them win better medals. Although some believe that this is against the spirit of the olympiad, many nations find it hard to justify leaving their best students at home.