The Columbia Course Directory and Vergil offer key information about each class including the course size, prerequisites, and if there are any restrictions. For example, if a course is not open to students in GSAS, it would not be open to an MA Statistics student. For further details about registration procedures, students may refer to the GSAS Registration page. 

Note that most courses in other departments are often restricted until the first week of the semester.  Courses that are not full by the first week of the semester will often open to MA Statistics students.  Keep checking if there is a particular course you are interested in.