Sal Khan discusses how mastery based learning is the best route for our students, using building a house to illustrate what happens if we do not teach math concepts through mastery. Would you give your contractor 2 weeks to build the foundation, regardless of external factors such as weather and supplies? Would ,you then, after two weeks, move the contractor along to build the first floor? Eventually, your house would be unstable by the end of building the first floor. This a is the same thing that is happening when we do not teach math concepts to full mastery.