Timed Writing

Because so much of our conversation seems to center around the writing of process pieces, I thought it might be interesting to explore the teaching of timed writings. I sometimes wonder what the purpose and benefit is of using timed writings as assessment. Is a timed writing an accurate assessment of a student's writing capabilities? So much of the standardized writing students do is based off of their ability to compose an essay in 1 hour. How much of our time should be dedicated to teaching this skill? Is there a way to teach timed writing that is not formulaic? Or is the formula a necessity for students to fall back on? What do you assess in a timed writing in comparison to a processed piece?