I don't have any solid evidence - I was assuming that the distribution for writing is not extremely different from the distributions for standardized math and reading scores, where the top 5% of one grade is well above average (50th percentile) for two grades up or more.
I would agree that a top 5th grader should be a much more proficient writer than a top 3rd grader. While I am confident that the written responses produced by DS and DD in third grade surpassed those of the average 5th grader, I am equally certain that they did not surpassed the top 5th graders.
However, I also realized that we are not talking about the same kind of writing. The reason why I was so impressed was because the writiing samples were insightful answers to literary analysis type questions completed independently in class under time constraints (10-15 minutes to analyze and produce the written responses).
The type of writing projects to which you refer are also common in our school but I don't tend to be impressed by them and the teachers don't normally give them much weight for grading purposes because it is not possible to control/determine how much input are provided by parents. I have to say that I have been shocked by how much of the work that some parents tell me they do for their children.
