# What is the relationship between scaled scores and percentile ranking in Assessment Series data?

When students’ scores are evenly distributed across the range of possible scaled scores (150-250), a student who scores at the 50th percentile will have a scaled score that matches the national average scaled score (All X Graders Nationally).

For example, if five students take the Assessment Series and earn scaled scores of 150, 175, 200, 225, and 250, their scaled scores are evenly distributed across the range of possible scores. The average scaled score for this group of students would be 200; a student scoring 200 would also be at the 50th percentile because they performed better than or the same as 50% of their classmates.

However, in reality, scores are typically unevenly distributed. This is especially true for the Assessment Series because our data displays update as more students submit each test. When more students are performing well (or poorly) on a test, the average scaled score will be higher (or lower) than the scaled score at the 50th percentile.

For example, let’s say that five students take an Assessment Series test and earn scaled scores of 150, 175, 200, 245, and 250. In this case, their scaled scores are no longer evenly distributed because two students have earned very high scores (245 and 250). These high scores would bring the average of the five students’ scaled scores up to 204. However, a student scoring 200 would still be at the 50th percentile because that student performed better than or the same as 50% of their classmates.

Similarly, five students with scaled scores of 150, 155, 200, 225, and 250 would also represent an uneven distribution, with two students earning very low scores (150 and 155). This would bring the average scaled score down to 196, but a student scoring 200 would remain at the 50th percentile, as in the scenario above.

It is also important to note that the average scaled score and student percentile rankings fluctuate because the data displays are updated as more students submit Assessment Series tests. However, each student’s scaled score remains the same over time. A student’s scaled score, on its own, is a valuable data point that indicates whether or not a student has demonstrated that they meet end-of-grade-level expectations for reading. In other words, educators can interpret students’ scaled scores regardless of national average scaled score and any student’s percentile ranking.