Inadequate sample sizes. Moreover, the testing needs to be done at older ages because all existing studies show that the longer a student stays in government schools the worse he does in relation to homeschoolers. This is almost certainly because as time goes on the influence of the home wanes in government school students.
Finally, there is very little unschooling, and it is mainly restricted to the hippy dippy left. Nevertheless, by the teen years, those kids are probably doing no worse than public school students.
We do not unschool, but to compare the results of structured vs. unschooling at an early stage demonstrates that the researchers were either dishonest or completely uninformed as how the method called “unschooling” works. With the unschooling method I am most familiar with, it is only typically at the age of ten or so that unschoolers start significantly absorbing data—but they do it at a massive rate. Compare the two groups’ SAT scores when they are finished homeschooling and then you might be able to discover some useful data.
No evidence cited of course, but they had to throw that old canard out there.