Sequenced prerequisite courses tend to increase the “weeding” effect by default. Calculus 2 instructors would not want students who passed calculus 1 to be unable to use calculus 1 concepts as needed in calculus 2, for example, so the rigor level of calculus 1 must be high enough to meet such a standard. External accreditation of the major can also have the effect of requiring a minimum level of rigor.