<p>
This reminds me of the following: (Yes, curm, I know I have quoted your sentence totally out of the context and apply it in another totally different context.)</p>
<p>This is especially true for the statistics for midterm grades. You really need to omit the bottom, say, 1/4. Why? This is because those kids in the bottom will drop the classes before the date the students are allowed to drop a course without leaving a “W” on the transcript. Before the final, all the bottom students are gone. If only 20 or 25 % will get an A, all of a sudden the standard for getting an A becomes higher.</p>
<p>So the average score and SD before the date the students could drop the course are very misleading.</p>