A year ago, I expressed my concerns on the Washington Post’s blog that your decimal place value videos and exercises failed to incorporate very basic knowledge about how people learn place value.
I wrote that your decimal comparison videos were problematic because they only addressed decimal numbers with the same number of decimal places, and that a very basic, robust finding in rational number learning research is that students do not struggle with these comparisons–because students can treat them like whole numbers and get correct answers. Instead, students struggle with comparisons where the decimals have different numbers of decimal places because here, the whole number place value rules do not apply.
Together with my co-author, I wrote,
A student who thinks that 0.435 > 0.76 is offered nothing in the way of correction on Khan Academy. In fact, one of the top questions on the page for this video (as of July 18, 2012) is “So is .02009 greater than .0207?” This is exactly the sort of question that a competent teacher of arithmetic needs to anticipate and to answer. Khan fails to pose it.
In short, these decimal videos and their accompanying exercises are useless.