[ippisl]

You're link implies that had the study done in a better way, it would have also shown the khan students imrpovement in pre-algebra , while not so for the control group , and were much more efficient in learning algebra:

""" It is also interesting to consider that students in the treatment group spent approximately half of the summer working on pre-algebra skills. Because the Khan software is individualized, it identified that most of our students had significant pre-algebra skill gaps and delivered instruction and practice problems to address these deficits. Students in the Khan/treatment group therefore spent up to 50% less time than the control group on the algebra content that the MDTP exam measured. The treatment group, however, still performed at a similar level the control group on the algebra measures. """

Of course this is just an hypothesis, and would need to be tested.