The author has discovered that errors in grades often occur when scores are combined for final marks. These errors are not related to the grading individual assignments. Rather, they occur when teachers at all grade levels bring individual test and assignment scores together for the students' final grades. Unfortunately, professors of mathematics and psychometrics have stood silently by and not alerted the teaching profession to the mathematical and measurement errors that are common when bringing scores together, making it difficult for teachers to find the information that would help them avoid these errors. In an attempt to learn more about this problem, the author conducted research comparing the effects of four methods of bringing scores together on class rank. He asked professors in various subject areas to submit raw scores for each test and class assignment from one of their classes and received responses for 37 classes. This article will explain the four errors and how to correct them.