There is growing evidence that the “linear” English languages courses are declining in usage in favor of more clustered materials and returning to the same basic sources when learning.
Fueled both by a demand for more efficient and faster delivery and more improvements being made constantly in the field there is a gradual shift being made from the traditional course structure which flows almost like a line, leading from one point to another consecutively. A “sequence” of education is presumed to have been followed by the students, allowing them to progress from stage to stage.
However as educational bodies learn more about the way young students learn language it has been found increasingly that as more sources and learning techniques become available teachers have greater potential to respond to the particular needs of their students and greater flexibility to tailor their classes in a way which would facilitate their pupils’ learning the English language.
See here for more on this.