Planning & Assessment: "Landmark Numbers": Error Analysis: Grade 4 Mathematics
Using "friendly numbers" to illustrate addition and subtraction is really a way of teaching estimation skills and later feeds students' ability to see if their answers to simple arithmetic operations make sense. This video shows two teachers poring over student work to figure out what might have been going on in their heads to make the mistakes they're making. This leads directly to re-teaching ideas. Good pictures of the actual student work accompanies hearing their dialog. Then they plan eactly how they are going to reteach the concept.
Other videos by related topic:
Skillful Data Use