A percentile rank explains how well a test-taker did in comparison to other test-takers, while a percentage rank explains how well a test-taker did on the test itself. For example, a student might answer 90 percent of questions correctly and rank in the 99th percentile. This means the student scored better than 99 percent of students who took the test.
Percentile ranks are used for tests where it is important to compare one test-taker's results to a large sampling of data. For example, doctors have special growth charts to rank children's height and weight by percentile. Children who fall into very high or very low percentiles are evaluated for health issues. Percentages are used in tests where individual performance is more important than relative performance. A doctor treating a child who has 40 percent lung function does not need to know the child's percentile.
Standardized tests often use percentiles to determine the best students. A very small number of students get perfect scores, or 100 percent accuracy, on the SAT. However, it is easier to determine percentile rank by looking at the total number of students who took the test, then divide that number by 100. Each year, the College Board releases data tables to show how percentile information corresponds to SAT test scores. A score of 770 or above in a single section generally ranks in the 99th percentile.