How well does Khan Academy teach?
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We contend that the videos at Khan Academy are alarmingly devoid of PCK (pedagogical content knowledge). We claim further that (1) the examples Khan chooses appear selected at random and thus are, perhaps unsurprisingly, often quite poor. They are prone either to create further confusion, or to fail to address fundamental questions students are likely to have; and (2) Khan’s explanations are frequently off target in addressing likely student questions that experienced teachers would anticipate and elicit.
We offer two examples. It is important to note that we did not choose these examples after watching hundreds of Khan’s videos. Instead, we came across them in the process of evaluating the usefulness of Khan’s video instruction for courses we teach, and for content in which we are interested. We contend that these examples are typical and that our critique applies broadly.
Comparing decimals. Decimal fractions (decimals for short — the numbers to the right of the decimal point) are a notoriously challenging topic in the elementary math curriculum. This difficulty is due at least in part to the fact that many rules that apply to whole numbers do not apply to decimals. Perhaps the most famous of these involves comparing numbers. When two whole numbers have different numbers of digits (e.g., 435 and 76), the one with more digits is greater (and usually by a lot!) This is not true of decimals: 0.435 is less than 0.76, despite having more digits.
Therefore, any attempt to help students improve their understanding of comparing decimals needs to deal with the case of decimals having different numbers of digits to the right of the decimal point. The standard American curricular treatment, in which students are instructed to append zeroes to the shorter decimal to equalize the number of digits helps students to perform this task correctly, but fosters its own misconceptions.
more . . . http://www.washingtonpost.com/blogs/answer-sheet/post/how-well-does-khan-academy-teach/2012/07/25/gJQA9bWEAX_blog.html#pagebreak
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